chore: downgrade compile target to Java 11

Uses <release>11</release> in maven-compiler-plugin (recommended over
source/target to guarantee bytecode and API surface match Java 11).

Code changes to drop Java 12-16 features:
- records (TeamRanking, PlayerRanking, internal tuples in
  SqliteTeamRepository) become hand-written immutable classes; same
  accessor names (rank()/team()/score()/...) so call sites are unchanged.
- instanceof X x pattern matching becomes classic instanceof + cast in
  CommandContext.requirePlayer and Database.normalize.
- switch expressions with -> arrows become classic switch + break, or
  if/else chains, in BaseCommand.handleResult, ArgumentTypes.BOOLEAN and
  TeamScoreSubCommand.execute.

docs/setup.md, features.md and decisions.md updated to reflect Java 11.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/setup.md

@@ -4,7 +4,7 @@
 
 - **Type** : librairie Java (`jar`) — pas un plugin Bukkit
 - **artifactId Maven** : `CR-Core`
-- **Build** : Maven, Java 16
+- **Build** : Maven, Java 11
 - **API serveur (provided)** : Paper 1.16.5
 - **SQLite (compile)** : `org.xerial:sqlite-jdbc:3.45.3.0`
 - **Package racine** : `fr.luc.crcore`