Merge branch 'stable-5.7' into stable-5.8

Merge branch 'stable-5.7' into stable-5.8

* stable-5.7:
  Remove unused imports
  Silence API warnings
  Remove erraneously merged source features
  Prepare 5.3.9-SNAPSHOT builds
  JGit v5.3.8.202011260953-r
  Prepare 5.1.15-SNAPSHOT builds
  JGit v5.1.14.202011251942-r
  GC#deleteOrphans: log warning for deleted orphaned files
  GC#deleteOrphans: handle failure to list files in pack directory
  Ensure that GC#deleteOrphans respects pack lock
  Update API warning filters
  Remove unused imports

Change-Id: I9b94938f5c09bd726e8e368c98c56da8280fd0b2
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>