Compile with Java 21

[mateuszkwiecinski]

blocked by https://issuetracker.google.com/issues/294137077

[github-actions]

Apk size:

Aar size change: +0.0B (33.9KiB -> 33.9KiB)

Diffuse output:

OLD: diffuse-source-file-release
NEW: shimmer-android-core-release.aar

 AAR      │ old      │ new      │ diff 
──────────┼──────────┼──────────┼──────
      jar │ 35.4 KiB │ 35.4 KiB │  0 B 
 manifest │    218 B │    218 B │  0 B 
      res │  1.4 KiB │  1.4 KiB │  0 B 
    other │  1.8 KiB │  1.8 KiB │  0 B 
──────────┼──────────┼──────────┼──────
    total │ 38.8 KiB │ 38.8 KiB │  0 B 

 JAR     │ old │ new │ diff      
─────────┼─────┼─────┼───────────
 classes │  17 │  17 │ 0 (+0 -0) 
 methods │ 251 │ 251 │ 0 (+0 -0) 
  fields │  79 │  79 │ 0 (+0 -0)