Independent table row spacing

You can make a tabular containing only one row, and break lines inside a cell using either \par as shown below, or \newline. As leandriis warned, horizontal lines created by commands from the booktabs package aren't designed to join well with vertical lines (the author of booktabs explains in the manual of his great package that vertical lines in tables are almost always a bad typographic choice: both ugly and useless).

\documentclass{article}
\usepackage{booktabs}

\begin{document}

\renewcommand{\arraystretch}{1.5}%
\begin{tabular}{ p{5cm} | p{5cm} }
  \toprule
  \textbf{Primäre Quellen}\par
  Jira\par
  Confluence\par
  E-Mail\par
  Nextcloud\par
  Rocket.Chat\par
  GitLab/GitHub &
  \textbf{Sekundäre Quellen}\par
  exply\par
  Canias ERP\par
  Diverse Excel-Tabellen (Vertrieb, Verwaltung)\\
  \bottomrule
\end{tabular}

\end{document}

You can get a nicer layout by removing the \renewcommand{\arraystretch}{1.5}, using a dedicated row for the table header, finishing it with \\\midrule and suppressing the vertical rule. As per leandriis' suggestion, I also added >{\raggedright\arraybackslash} in front of the second column specification in the tabular preamble, so that interword spacing in the second column doesn't get overstretched (this way, it is not stretched at all; as a consequence, the right side of the second column is allowed to have a “ragged” appearance, which doesn't change much here since we were ending lines/paragraphs manually anyway). The >{...} syntax requires the array package, hence we are adding it too.

\documentclass{article}
\usepackage{array}
\usepackage{booktabs}

\begin{document}

\begin{tabular}{ p{5cm} >{\raggedright\arraybackslash} p{5cm} }
  \toprule
  \textbf{Primäre Quellen} & \textbf{Sekundäre Quellen}\\
  \midrule
  Jira\par
  Confluence\par
  E-Mail\par
  Nextcloud\par
  Rocket.Chat\par
  GitLab/GitHub &
  exply\par
  Canias ERP\par
  Diverse Excel-Tabellen (Vertrieb, Verwaltung)\\
  \bottomrule
\end{tabular}

\end{document}

You could also do that using multicols (possibly inside a minipage) and/or enumitem. There are many possibilities.

P.S.: as Mico said, if you use \caption and \label, be sure to put \label after the associated \caption, because it is \caption that increases the counter (\label uses the last reference set with \refstepcounter)!

Use two distinct tabular environments for the columns:

\documentclass[a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[ngerman]{babel}
\usepackage{array,booktabs}

\begin{document}

\begin{table}[htp]
\centering

\begin{tabular}{ll}
\toprule
\textbf{Primäre Quellen} & \textbf{Sekundäre Quellen} \\
\midrule
  \begin{tabular}[t]{@{}>{\raggedright\arraybackslash}p{5cm}@{}}
  Jira \\
  \addlinespace
  Confluence \\
  \addlinespace
  E-Mail \\
  \addlinespace
  Nextcloud \\
  \addlinespace
  Rocket.Chat \\
  \addlinespace
  GitLab/GitHub \\
  \end{tabular}
&
  \begin{tabular}[t]{@{}>{\raggedright\arraybackslash}p{5cm}@{}}
  exply \\
  \addlinespace
  Canias ERP \\
  \addlinespace
  Diverse Excel-Tabellen (Vertrieb, Verwaltung) \\
  \end{tabular}
\\
\bottomrule
\end{tabular}

\caption{Primäre und sekundäre Quellen bei XXXXXX}
\label{table:informationsquellen}

\end{table}

\end{document}

Be careful that \label should go after \caption. Using \arraystretch yields not very nice spaces, better using \addlinespace where needed.

To prevent the material in the two columns from interacting, you could place it into separate subordinated tabular environments. In the following solution, the "outer" tabular environment consists of two l columns; the outer environment is needed solely for the \toprule and \bottomrule directives. The "inner" tabular environments each contain a single p column, which allows automatic line-breaking (if needed).

I would also omit the vertical divider line.

\documentclass{article}
\usepackage{booktabs}

\begin{document}
\begin{table}[h!]
\renewcommand{\arraystretch}{1.5}
\centering
\footnotesize % is this really needed?
\begin{tabular}{ ll }  % "outer" tabular
  \toprule
  \begin{tabular}[t]{@{} p{5cm} @{}} % first "inner" tabular
    \textbf{Primäre Quellen} \\
    Jira        \\                  
    Confluence  \\              
    E-Mail      \\                             
    Nextcloud   \\               
    Rocket.Chat \\            
    GitLab/GitHub 
  \end{tabular} &          
  \begin{tabular}[t]{@{} p{5cm} @{}} % second "inner" tabular
    \textbf{Sekundäre Quellen} \\
    exply      \\
    Canias ERP \\
    Diverse Excel-Tabellen (Vertrieb, Verwaltung) 
  \end{tabular}\\
  \bottomrule
\end{tabular}
\caption{Primäre und sekundäre Quellen bei XXXXXX}
\label{table:informationsquellen}
\end{table}
\end{document}