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.
• Removed CHANGES.txt. Developers should maintain the changelog via the GitHub tag/release UI, which serves as the source of truth for version numbers and provides a 'description' box for changes. The previous process of tracking the changelog required duplicating the same information in both CHANGES.txt and the tag/release UI.
• 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.

Upon review and acceptance, I will also make some changes to the wording in the PR template which still includes references to the changelog:

**Related Issue(s):**

Closes #


**Proposed Changes:**

For inclusion in changelog (if applicable):

1.

Not intended for changelog:

1. 

**Diff of User Interface**

Old behavior:

New behavior:




**PR Checklist:**

- [] This PR is as small and focused as possible
- [] If this PR includes proposed changes for inclusion in the changelog, the title of this PR summarizes those changes and is ready for inclusion in the Changelog.
- [] I have updated docstrings for function changes and docs in the 'docs' folder for user interface / behavior changes
- [] This PR does not break any examples or I have updated them

**(Optional) @mentions for Notifications:**