Bump fabric-loom from 1.13-SNAPSHOT to 1.15.0-alpha.6

Bumps fabric-loom from 1.13-SNAPSHOT to 1.15.0-alpha.6.

---
updated-dependencies:
- dependency-name: fabric-loom
  dependency-version: 1.15.0-alpha.6
  dependency-type: direct:production
...

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

Author: dependabot[bot]

gradle/libs.versions.toml

@@ -18,7 +18,7 @@ neoforgeVersion = "21.11.0-beta"
 modmenu-version = "17.0.0-beta.1"
 clothConfigVersion = "20.0.149"
 
-fabric-loom-version = "1.13-SNAPSHOT"
+fabric-loom-version = "1.15.0-alpha.6"
 forge-plugin-version = "[6.0.46,6.2)"
 neoforge-plugin-version = "2.0.134"
 curse-version = "1.4.0"