Bump github.com/sagernet/sing from 0.2.14 to 0.2.15

Bumps [github.com/sagernet/sing](https://github.com/sagernet/sing) from 0.2.14 to 0.2.15.
- [Commits](https://github.com/sagernet/sing/compare/v0.2.14...v0.2.15)

---
updated-dependencies:
- dependency-name: github.com/sagernet/sing
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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