Update gradle and git checkout

[koppor]

• New patch level of gradle: https://docs.gradle.org/6.2.2/release-notes.html
• We do not use git submodules any more - we moved to git subtree at Switch to git subtree #5647 - Thus, we can use the latest checkout source action (https://github.com/actions/checkout#whats-new)

In GitVersion we nevertheless fetch the whole history, so it does not harm to go for the latest version of checkout.

[codecov]

Codecov Report

Merging #6090 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #6090      +/-   ##
============================================
- Coverage     41.52%   41.52%   -0.01%     
+ Complexity     6873     6872       -1     
============================================
  Files          1092     1092              
  Lines         46342    46342              
  Branches       5563     5563              
============================================
- Hits          19245    19242       -3     
- Misses        25650    25652       +2     
- Partials       1447     1448       +1

Impacted Files	Coverage Δ	Complexity Δ
...bref/logic/remote/server/RemoteListenerServer.java	70.96% <0%> (-6.46%)	7% <0%> (ø)
...bref/logic/importer/fileformat/MrDLibImporter.java	83.07% <0%> (-1.54%)	22% <0%> (-1%)

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[koppor]

Works great

Thus, I merge (to keep us focused on the other PRs)