Update official release to v8.6.9

[fharbe]

UI-Glenn has promoted UniFi Network Application 8.6.9 to Stable/Official yesterday.

Changes proposed in this pull request:

• Create 8.6/official, v8.6.9
• Promote 8.6 official to latest
• Tag 8.6.9 release
• Create GitHub Actions workflow build-8.5-official

@goofball222