Raise baseline Java version to 21

[nscuro]

Description

Raises the baseline Java version to 21.

This allows us to make use of new language features introduced since Java 17.

We've been shipping container images with JRE 21 since DT v4.10.0.

Addressed Issue

N/A

Additional Details

N/A

Checklist

• I have read and understand the contributing guidelines
• This PR fixes a defect, and I have provided tests to verify that the fix is effective
• This PR implements an enhancement, and I have provided tests to verify that it works as intended
• This PR introduces changes to the database model, and I have added corresponding update logic
• This PR introduces new or alters existing behavior, and I have updated the documentation accordingly

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 16cfd7c1	✅ 100.00% (target: 70.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (16cfd7c)	Report Missing	Report Missing	Report Missing
Head commit (63d9021)	22058	16750	75.94%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#3682)	1	1	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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_{Codacy will stop sending the deprecated coverage status from June 5th, 2024. Learn more}

Footnotes

1. Codacy didn't receive coverage data for the commit, or there was an error processing the received data. Check your integration for errors and validate that your coverage setup is correct. ↩