chore(ci): automate publishing again

[matthewelwell]

Thanks for submitting a PR! Please check the boxes below:

• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?

Changes

This PR turned out to be way more painful than I expected. The original goal was to revert this PR, which was added before we had release please to group releases. The key issue though is that release please and helm's chart-releaser-action do not work well together as they both want to create the github release and tag.

Useful references:

• How to use this action with Googl's "release-please" helm/chart-releaser-action#159
• https://github.com/Mailu/helm-charts/blob/master/.github/workflows/publish-chart.yaml

The new workflow steps added in this PR (generated with help from ChatGPT) essentially replace the work of the chart releaser action. The new flow is:

1. Release please release PR is merged
2. Release please action creates the github release and tag (in the format flagsmith-X.Y.Z for consistency)
3. Release workflow is triggered on tag creation
4. Release workflow builds the chart binary and uploads to the release created by release please
5. Release workflow updates the gh-pages branch with new meta information on the release
6. Change to gh-pages branch automatically triggers a rebuild of the GH pages site (which acts as the helm repo for flagsmith charts)

How did you test this code?

I've tested this by manually creating the flagsmith-0.75.0 tag from this branch, but the real test will be once we merge the next release please PR.

[emyller]

🎉

#398 (comment)

[emyller]

Nice job