Clean up / simplify CI

[twyatt]

• Updates GitHub virtual environments to macOS 12 (from 11)
• Uses gradle/gradle-build-action@v2 for Gradle execution (to get its caching support, and remove the manual caching setup)
• Skips local maven publish on forks

[github-actions]

Test Results

  50 files    50 suites   31s ⏱️
252 tests 252 ✔️ 0 💤 0 ❌
297 runs  297 ✔️ 0 💤 0 ❌

Results for commit 59e7c44.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

Merging #246 (59e7c44) into main (8c86321) will increase coverage by 8.62%.
The diff coverage is 70.88%.

@@             Coverage Diff              @@
##               main     #246      +/-   ##
============================================
+ Coverage     70.47%   79.10%   +8.62%     
- Complexity       67       90      +23     
============================================
  Files            31       25       -6     
  Lines           918     1254     +336     
  Branches         83      124      +41     
============================================
+ Hits            647      992     +345     
+ Misses          236      203      -33     
- Partials         35       59      +24     

Impacted Files	Coverage Δ
color/src/commonMain/kotlin/NextColor.kt	75.00% <ø> (ø)
kanvas/src/commonMain/kotlin/svg/PathString.kt	0.00% <0.00%> (-43.59%)	⬇️
shape/src/commonMain/kotlin/curve/Linear.kt	0.00% <0.00%> (ø)
kanvas/src/commonMain/kotlin/Clip.kt	41.17% <23.07%> (-18.83%)	⬇️
kanvas/src/commonMain/kotlin/Ellipse.kt	81.25% <25.00%> (-4.47%)	⬇️
kanvas/src/commonMain/kotlin/Transform.kt	72.97% <42.85%> (-11.41%)	⬇️
...vas/src/commonMain/kotlin/svg/PathStringBuilder.kt	56.00% <56.00%> (ø)
kanvas/src/commonMain/kotlin/SegmentedPath.kt	58.76% <58.76%> (ø)
color/src/commonMain/kotlin/Color.kt	89.47% <83.33%> (+1.23%)	⬆️
kanvas/src/commonMain/kotlin/Path.kt	83.33% <83.33%> (ø)
... and 10 more

... and 4 files with indirect coverage changes