Version bump to 3.5.15 (#6514)

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

redbot/__init__.py

@@ -339,7 +339,7 @@ def _early_init():
 
 
 # This is bumped automatically by release workflow (`.github/workflows/scripts/bump_version.py`)
-_VERSION = "3.5.15.dev1"
+_VERSION = "3.5.15"
 
 __version__, version_info = VersionInfo._get_version()