x/build/perf/app/dashboard: automate the branch list

[findleyr]

I started working on this today as a warmup, then realized I had bitten off more than I could chew, so am instead filing a reminder issue for the next team-wide fixit:

It would be nice if:

1. The branch list here were not hard-coded, and
2. There were an alias for "latest release branch" in the query string, for example ?branch=release.

In addition to avoiding a maintenance task every release cycle, this would allow us to have a permalink to x/tools/gopls benchmarks, which only run on the latest release. (right now, I have a Google internal short link that I keep updating).

Never a big enough deal to prioritize, but apparently a manual task performed by multiple team members! A good candidate for a fixit week.

[dmitshur]

Noting here that the maintner service's ListGoReleases API call might be useful for this.

Sample maintq list-releases output

$ go run golang.org/x/build/maintner/maintq@latest list-releases
releases: <
  major: 1
  minor: 22
  patch: 3
  tag_name: "go1.22.3"
  tag_commit: "adbfb672ba485630d75f8b5598228a63f4af08a4"
  branch_name: "release-branch.go1.22"
  branch_commit: "185457da9bf7b617565e05e4ba95d89095fe7613"
>
releases: <
  major: 1
  minor: 21
  patch: 10
  tag_name: "go1.21.10"
  tag_commit: "752b009010df021c45f620e683ec062d22b552bf"
  branch_name: "release-branch.go1.21"
  branch_commit: "6f5219571c5fde780573918ba334030f002ce00e"
>