helm: cleanup helm chart, now in mastodon/chart

[consideRatio]

• This is a followup to Release helm chart to github pages on pushes to main #20427 where it was concluded that the Helm chart should be relocated to a dedicated repository. The Helm chart is now maintained in https://github.com/mastodon/chart.

The following PRs are open and need to be re-created in that repository as it should no longer be tracked here.

• Closes Chart fixes/enhancements #21691
• Closes fix(helm-chart): resolve template rendering error in sidekiq deployment #21649
• Closes chart: fix double-unwrap of .Values.mastodon.s3.alias_host #21623
• Closes Helm: fix assets pvc values #21549
• Closes Helm: Small fixes for helm chart #21508
• Closes chart: fix nesting of redis password placeholder #21430
• Closes chart: add a toggle to opt-out of the rollme pod annotation #21391
• Closes Add the possibility to set PREPARED_STATEMENTS environment variable #21261
• Closes Add Helm Chart values for various Puma related settings #21053
• Closes Support for setting redis-password via Helm chart for external redis instance #20819
• Closes allow the oidc client secret to be set in an existing secret in helm chart #20803

EDIT: I incorrectly added two PRs here related to docker-compose and puma.rb that I've removed. Sorry for the noise!

[Eriner]

This PR does not close #21165, as the docker-compose.yml file is not changed in this PR. Please do not close that issue if/when merging this PR.

[jimeh]

This PR does not supersede #21048 which modifies config/puma.rb.

[consideRatio]

@Eriner and @jimeh thank you for helping me realize that! Sorry for the noise, I've updated the description to not close those two PRs.

[consideRatio]

@Gargron I understand it as you relocated the helm chart, can you review this as well?