Update for Go 1.21

What does this MR do and why?

Update for Go 1.21

- Updates linter to Go 1.21
- Updates tools version to Go 1.21.6

Closes https://gitlab.com/gitlab-org/gitlab-zoekt-indexer/-/issues/31

Signed-off-by: Robert Marshall <rmarshall@gitlab.com>

Closes #31 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.