x/build/cmd/releasebot: for major releases, also check the first minor release milestone exists

[dmitshur]

I noticed that GopherBot hadn't closed #44346 even though CL 293411 was submitted. I figured out the problem by looking at its logs and seeing:

2021/02/19 00:26:09 Failed to apply minor release milestone to issue 44346: no minor milestone found for release series 1.16
2021/02/19 00:26:09 Failed to apply minor release milestone to issue 44358: no minor milestone found for release series 1.16
2021/02/19 00:26:09 Failed to apply minor release milestone to issue 44402: no minor milestone found for release series 1.16

GopherBot failed to apply a minor release milestone to the issue, and so the issue's milestone didn't match the CL's branch goX.Y version. GopherBot didn't apply the minor release milestone because the milestone didn't exist.

I fixed the immediate issue by creating the Go1.16.1 milestone.

I'll also send a CL to add the missing step of creating the next major and minor milestone to our internal release process, so that this won't happen again for future major Go releases.

CC @golang/release, @ianlancetaylor.

[gopherbot]

Change https://golang.org/cl/294249 mentions this issue: cmd/releasebot: add check for "create first minor release milestone" step