This pull request introduces several enhancements and updates across various files to improve workflows, templates, and repository configurations. Key changes include the addition of GitHub Actions workflows for automation, updates to pull request templates, and adjustments to repository ownership and funding configurations.

GitHub Actions Workflows:

• Added .github/workflows/ami-release-nix.yml and .github/workflows/ami-release-nix-single.yml for automating AMI builds with PostgreSQL versions, including multi-stage builds and artifact uploads to S3. [1] [2]
• Introduced .github/workflows/check-shellscripts.yml to validate shell scripts using ShellCheck.
• Added .github/workflows/ci.yml to enforce merge requirements by validating PostgreSQL release version formats.

Pull Request Templates:

• Created default.md and extension_upgrade.md templates for pull requests to standardize information provided during PR creation. [1] [2]
• Updated .github/pull_request_template.md to guide contributors to select appropriate sub-templates.

Repository Configuration:

• Updated .github/CODEOWNERS to assign ownership of specific files and directories to relevant teams.
• Modified .github/FUNDING.yml to reflect the new GitHub Sponsors username tealbase.

Miscellaneous:

• Updated .envrc.recommended to include watch_file and use flake for Nix shell environments.
• Added .github/actions/shared-checkout/action.yml for a reusable GitHub Action to checkout repositories in workflows.