Bump Kotlin to 2.2.20 and clean up build scripts

[lppedd]

This PR contains multiple changes:

• Updates Kotlin to 2.2.20-RC2.
• Updates Dokka to 2.1.0-Beta.
• Replaces Nexus Publish plugin with Vanniktech's plugin.
  See the release.yml workflow, and comments in build.gradle.kts. You will have to fill in the TODO and test that everything works.
• Simplifies the JaCoCo configuration. The convention plugin was too much imo, as everything fits nicely in the main script.
• Updates the Reckon plugin to 1.0.0 and enables Gradle's configuration cache.

Note: in draft until 2.2.20 is released.

Closes: #148
Closes: #113

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.23%. Comparing base (15abe10) to head (b9de774).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #149      +/-   ##
============================================
+ Coverage     85.21%   85.23%   +0.02%     
- Complexity      218      219       +1     
============================================
  Files            29       29              
  Lines           656      657       +1     
  Branches        117      117              
============================================
+ Hits            559      560       +1     
  Misses           68       68              
  Partials         29       29              

Flag	Coverage Δ
unittests	85.23% <ø> (+0.02%)	⬆️

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[lppedd]

Another question: does it really make sense to keep Detekt? Asking as it's an added complication for little value.
The codebase is relatively small, and PR reviews will catch any style issue.

[lppedd]

@petertrr ready to go whenever you have the time.