Prepare first OSS release v0.2.2

[pinodeca]

Prepares pg_durable v0.2.2 for OSS release.

Summary:

• Add v0.2.2 release notes and public OSS project metadata.
• Rename the license file to LICENSE.txt and align package metadata with the PostgreSQL license.
• Add third-party NOTICE content and PostgreSQL license headers across source-like files.
• Sanitize ACR deployment defaults and prompt references for public release.

Validation:

• git diff --check main...HEAD
• cargo metadata --format-version 1 --no-deps
• bash -n scripts/deploy-acr.sh
• make -n docker-push
• cargo fmt -p pg_durable -- --check