Simplify Release Process

[asonnenschein]

This PR simplifies the SDK release process by reducing the number of release branches, naming conventions, and number of steps to release:

• All releases are now tagged from the main branch. Developers should still create a release branch, but only to bump the version number and merge to main. The previous release process required tagging releases from release branches, and making multiple updates to the release version number both before and after tagging the release.
• Included new workflow for releasing via gh CLI.
• Abandon release naming conventions. Developers should follow vanilla semantic versioning when tagging releases. The previous release process included naming convention prefixes/suffixes like maint-1.0.0 and 1.0.0dev.
• Remove internal Planet Slack announcement steps. I am currently working with folks to integrate GitHub notifications from this repository with our internal company Slack, which will announce releases automatically as they are tagged.

This MR doesn't make any changes to automated CI workflows used in the release process; the changes proposed here should not have any side effects with existing CI workflows.

[asonnenschein]

Made some updates, per feedback during office hours:

• Updated the release docs with a workflow for tagging a new release via the gh CLI
• Added CHANGES.txt back
• Updated Versions and Stability section in README with Ian's text, and updated the Versions and Stability section in RELEASES with a simple note about semantic versioning.