build: fix maven-enforcer config and enforce plugin versions

[spannm]

Summary

According to Official Apache Maven documentation, Maven 3.8.x and earlier have reached End of Life (EOL) and new plugin releases will require Maven 3.9.0 or later.

This PR updates our build constraints accordingly, raises the minimum required Maven version to 3.9, and fixes a misconfiguration within the maven-enforcer-plugin.

Changes

• Drop EOL Maven Support: Bumped minimum Maven version constraint from [3.6,) to [3.9,) within requireMavenVersion.
• Fix Enforcer Configuration: Moved the requireMavenVersion rule inside the <rules> element to resolve the unknown parameter warning.
• Enforce Plugin Versioning: Added the requirePluginVersions rule to guarantee reproducible builds by banning missing plugin versions, excluding common core plugins via unCheckedPluginList.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.48%. Comparing base (bd917e9) to head (71dfb2c).
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Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1867      +/-   ##
============================================
- Coverage     92.52%   92.48%   -0.05%     
+ Complexity     3563     3561       -2     
============================================
  Files           346      346              
  Lines          7050     7050              
  Branches        684      684              
============================================
- Hits           6523     6520       -3     
- Misses          365      366       +1     
- Partials        162      164       +2     

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[kingthorin]

Should the project's maven wrapper be updated as well?

(I have no idea how that's handled, my primary projects use gradle)

[spannm]

Should the project's maven wrapper be updated as well?

Yes @kingthorin, I just updated the wrapper scripts and properties using ./mvnw wrapper:wrapper.
The Maven version itself in .mvn/wrapper/maven-wrapper.properties was already up-to-date, but the underlying wrapper infrastructure had updates available. I've pushed the changes in a separate commit.

[kingthorin]

Looks good to me