Rework spellchecking, add lint-spell (#29112)

Backport clean cherry-picks of
9c39f8515f
and
c7a21cbb0c
onto 1.21.

- Use maintained fork https://github.com/golangci/misspell
- Rename `mispell-check` to `lint-spell`, add `lint-spell-fix`
- Run `lint-spell` in separate actions step
- Lint more files, fix discovered issues
- Remove inaccurate and outdated info in docs (we do not need GOPATH for
tools anymore)

Maybe later we can add more spellchecking tools, but I have not found
any good ones yet.

templates/repo/branch_dropdown.tmpl

@@ -1,7 +1,7 @@
 {{/* Attributes:
 * root
 * ContainerClasses
-* (TODO: search "branch_dropdown" in the template direcotry)
+* (TODO: search "branch_dropdown" in the template directory)
 */}}
 {{$defaultSelectedRefName := $.root.BranchName}}
 {{if and .root.IsViewTag (not .noTag)}}