Update jetty jspc maven plugin to 10.0.21

[merks]

FYI, I had to increment the version after the last bump:

https://github.com/eclipse-platform/eclipse.platform/pull/1409

This time likely requires a forced qualifier change.

[github-actions[bot]]

Test Results

1 156 files   -   578  1 156 suites   - 578   53m 12s :stopwatch: - 33m 59s 3 967 tests ±    0  3 922 :white_check_mark:  -    23   45 :zzz: +23  0 :x: ±0  8 332 runs   - 4 166  8 218 :white_check_mark:  - 4 119  114 :zzz:  - 47  0 :x: ±0 

Results for commit 9ca89c46. ± Comparison against base commit 7ff7a582.

This pull request skips 23 tests.

``` AutomatedResourceTests AllAliasTests BasicAliasTest ‑ testBug198571 AutomatedResourceTests AllFileSystemTests SymlinkTest ‑ testSymlinkPutHidden AutomatedResourceTests AllLocalStoreTests MoveTest ‑ testMoveFileAcrossVolumes AutomatedResourceTests AllLocalStoreTests MoveTest ‑ testMoveFolderAcrossVolumes AutomatedResourceTests AllRegressionTests Bug_025457 ‑ testFile AutomatedResourceTests AllRegressionTests Bug_025457 ‑ testFolder AutomatedResourceTests AllRegressionTests Bug_025457 ‑ testProject AutomatedResourceTests AllRegressionTests Bug_026294 ‑ testDeleteClosedProjectWindows AutomatedResourceTests AllRegressionTests Bug_026294 ‑ testDeleteFolderWindows AutomatedResourceTests AllRegressionTests Bug_026294 ‑ testDeleteOpenProjectWindows … ```

[akurtakov]

Jenkins succeeded. Windows GH action failed without even managing to start thus irrelevant for this PR.