Bump net.fabricmc:sponge-mixin

Bumps [net.fabricmc:sponge-mixin](https://github.com/FabricMC/Mixin) from 0.15.4+mixin.0.8.7 to 0.15.5+mixin.0.8.7.
- [Commits](https://github.com/FabricMC/Mixin/commits)

---
updated-dependencies:
- dependency-name: net.fabricmc:sponge-mixin
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

gradle/libs.versions.toml

@@ -1,6 +1,6 @@
 [versions]
 fabric_loader_version = "0.16.8"
-mixin_version = "0.15.4+mixin.0.8.7"
+mixin_version = "0.15.5+"
 asm_version = "9.7"
 commons_lang_version = "3.14.0"