Update deps and fix `go mod tidy` (#1514)

## Buildtools PR checklist

- [x] The code in this PR is covered by unit/integration tests.
- [x] I have tested these changes and provide testing instructions
below.
- [x] I have either responded to, or resolved all Gemini comments on the
PR.
- [x] I have read Google Eng Practices on [Small
Changes](https://google.github.io/eng-practices/review/developer/small-cls.html),
this PR either follows these guidelines or the description provides
reasoning for why they can not be followed.

## Description

* Bump Bazel, rules_go, gazelle and Go version
* Add missing generating code
* Make go.mod the source of truth for Bazel builds and tidy it
* Ensure go.mod/go.sum remain tidy and `go build ./...` passes via a CI
job
* Drop the EoL'd ubuntu-20.04 run

Fixes https://github.com/bazel-contrib/buildtools/issues/1513