Add step for merging release branch back to main.

[pstibrany]

This PR updates release procedure to include merging of release branch to main (after each release). This makes the latest version tags reachable from main, which is important for Go modules to not get confused about which version to use when trying to use branch names. This is only important for projects (eg. Cortex) that depend on Thanos.

For example:

$ go get github.com/thanos-io/thanos@main
go get: github.com/thanos-io/thanos@main (v0.19.1-0.20211012092857-7dee6fa15682) requires github.com/thanos-io/[email protected], not github.com/thanos-io/thanos@main (v0.19.1-0.20211012092857-7dee6fa15682)

This happens because latest reachable tag from main branch is v0.19.0 and not v0.23.1.

[yeya24]

The step looks good! But the format check still fails...

[pstibrany]

Thanks for ping, I forgot about this. I've pushed formatted docs. Hopefully it will work.

[pstibrany]

Now the failure seems to be unrelated to my change:

error: 		docs/getting-started.md:102: "https://build.thebeat.co/monitoring-the-beat-microservices-a-tale-of-evolution-4e246882606e" not accessible even after retry; status code 0: Get "https://build.thebeat.co/monitoring-the-beat-microservices-a-tale-of-evolution-4e246882606e": net/http: request canceled (Client.Timeout exceeded while awaiting headers)