Merge day - GeckoView Release (70, 71, 72)

[pocmo]

Merge day is coming up again! 👍

Merge day: Monday, October 21st
Release day: Tuesday, October 22nd

• browser-engine-gecko-beta (70) -> browser-engine-gecko-release (70)
• browser-engine-gecko-nightly (71) -> browser-engine-gecko-beta (71)
• browser-engine-gecko-nightly -> 72

┆Issue is synchronized with this Jira Task

[pocmo]

The last time I took notes to write a documentation for merge day (#3739). This time I am in Toronto at merge day so I'll try to find a "victim" to do merge day with the documentation and my help. :)