Bump Bannerlord.ButterLib from 2.8.13 to 2.8.15

Bumps [Bannerlord.ButterLib](https://github.com/BUTR/Bannerlord.ButterLib) from 2.8.13 to 2.8.15.
- [Release notes](https://github.com/BUTR/Bannerlord.ButterLib/releases)
- [Changelog](https://github.com/BUTR/Bannerlord.ButterLib/blob/dev/changelog.txt)
- [Commits](BUTR/Bannerlord.ButterLib@v2.8.13...v2.8.15)

---
updated-dependencies:
- dependency-name: Bannerlord.ButterLib
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Directory.Build.props

@@ -6,7 +6,7 @@
     <DepotName>public</DepotName>
     <HarmonyVersion>2.2.2</HarmonyVersion>
     <MCMVersion>5.9.1</MCMVersion>
-    <ButterLibVersion>2.8.13</ButterLibVersion>
+    <ButterLibVersion>2.8.15</ButterLibVersion>
     <TargetFramework>net472</TargetFramework>
     <LangVersion>8.0</LangVersion>
     <GameFolder>$(SolutionDir)\bin\bannerlord-$(GameBranch)</GameFolder>