Replace player-animation-lib with the Saturn emote library

- Every version depends on org.saturnclient:emotes-fabric
  (${emotes_version}+<mc>, bundled). playEmote plays Saturn's emotes
  through it, and sneaking stops your own emote and tells the server.
  Emotes now work on 1.21.6+ too, where player-animation-lib was
  commented out.
- Removed every player-animation-lib and bendy-lib reference: the KoSmX
  repository, the dependencies, the player_anim properties, and the old
  (partly commented out) code in SaturnProviderImpl and KeyInputHandler.
- Build: Loom 1.15 -> 1.17.21 (the emotes jar is built with 1.17 and
  older Loom refuses it), Gradle 9.3 -> 9.8 (needed by Loom 1.17), and
  sourceSets moved out of the dependencies block in the root build.gradle,
  which Gradle 9.8 rejects.
- common: points at the emote-library branch (saturnclientmc/common#5).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

versions/1.21.8/gradle.properties

@@ -10,12 +10,10 @@ org.gradle.configuration-cache=false
 minecraft_version=1.21.8
 yarn_mappings=1.21.8+build.1
 loader_version=0.19.0
-loom_version=1.15-SNAPSHOT
+loom_version=1.17.21
 
 fabric_api_version=0.136.1+1.21.8
 
 # Mod Properties
 maven_group=org.saturnclient
 archives_base_name=saturn-client
-
-# player_anim = 2.0.2+1.21.5