Bump org.spongepowered:mixin from 0.8.5 to 0.8.7

Bumps [org.spongepowered:mixin](https://github.com/SpongePowered/Mixin) from 0.8.5 to 0.8.7.
- [Release notes](https://github.com/SpongePowered/Mixin/releases)
- [Commits](SpongePowered/Mixin@releases/0.8.5...releases/0.8.7)

---
updated-dependencies:
- dependency-name: org.spongepowered:mixin
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

build.gradle.kts

@@ -65,7 +65,7 @@ dependencies {
     modCompileOnly("cc.polyfrost:oneconfig-$platform:0.2.0-alpha+")
 
     if (platform.isLegacyForge) {
-        compileOnly("org.spongepowered:mixin:0.8.5")
+        compileOnly("org.spongepowered:mixin:0.8.7")
         shade("cc.polyfrost:oneconfig-wrapper-launchwrapper:1.0.0-beta+")
     }
 }