Upgrade dependencies, SpotBugs, PMD, CheckStyle

[holgerfriedrich]

I have put all upgrades in one PR - as separate commits - to ease comparing the results before / after.
Pls. DO NOT SQUASH.

• Upgrades Checkstyle from 10.17.0 to 10.21.1.

For release notes, see:
https://github.com/checkstyle/checkstyle/releases

Note that default behavior for checking tags in javadoc changed
in version 10.20.2, it will not longer issue a warning about
missing tags if there is no javadoc section at all.
See https://github.com/checkstyle/checkstyle/issues/11584

• Upgrades PMD from 7.8.0 to 7.9.0

For release notes, see:
https://github.com/pmd/pmd/releases/tag/pmd_releases%2F7.9.0

• Upgrades spotbugs-maven-plugin from 4.8.6.2 to 4.8.6.6.

For release notes, see:
https://github.com/spotbugs/spotbugs-maven-plugin/releases

• Upgrade dependencies and plugins

* junit to 5.11.4
* commons-io to 2.18.0
* commons-lang3 to 3.17.0
* maven-plugin-api to 3.9.9
* maven-plugin-annotations 3.15.1
* mojo-executor to 2.4.1
* jsoup to 1.18.3
* jdt-annotations to 2.3.100
* flexmark to 0.64.8
* logback to 1.2.13
* plexus-compiler-eclipse to 2.15.0
* ecj to 3.39.0
* truth to 1.4.4

---

Notes:

• Error processing opcode ldc_w @ 14 in jdk.nashorn.internal.objects.NativeArray.$clinit$ spotbugs/spotbugs#1894 is still there in 4.9.0, and seen in our add-ons build in js automation.
• spotbugs 4.9.0 runs into OOM in googlestt binding, that's why I stick to 4.8.6. spotbugs-maven-plugin is anyway not yet available as 4.9.x.y release
• We still stick to Java 11, see discussion here. This starts blocking dependency upgrades and should be resolved.

[holgerfriedrich]

The results of my overnight build of add-ons repo with 0.18-SNAPSHOT look similar to the report by Jenkins CI with 0.17. I had 4 more warnings with the new one, bit might be on a slightly different commit.