[CALCITE-4147] Rename "master" branch to "main"

[michaelmior]

This would need to happen in concert with changes to the Calcite repo.
Although I think perhaps the first (and lowest impact) change will be to the site repo.

Note: This PR is merely to help with review. It should not be merged as it will eventually replace master

[F21]

Hey @michaelmior, thanks so much for taking care of this.
Just a few small things that appear to be missed:

• https://github.com/apache/calcite-avatica/blob/master/.github/workflows/publish-non-release-website-updates.yml#L22
• https://github.com/apache/calcite-avatica/blob/master/.github/workflows/publish-non-release-website-updates.yml#L57 (calcite-site was also renamed to main)
• https://github.com/apache/calcite-avatica/blob/master/.github/workflows/publish-site-and-javadocs-on-release.yml#L52 (calcite-site was renamed to main)
• https://github.com/apache/calcite-avatica/blob/master/site/README.md?plain=1#L29
• https://github.com/apache/calcite-avatica/blob/master/site/_docs/index.md?plain=1#L28 (Julian renamed his main branch to main so the link works due to redirection from master, but we might as well update it)
• https://github.com/apache/calcite-avatica/blob/master/site/develop/avatica.md?plain=1#L94 (the link is a 404, so maybe we can just remove it?)

[michaelmior]

@F21 Updated. As for the broken Jenkins link, I just removed that whole (small) subsection. I don't think it's really needed.

[F21]

LGTM! Thanks @michaelmior !