Update proposals to reflect implementation status, Swift version, and forum discussion links

[stmontgomery]

This updates some details in the accepted proposals in the repository, to better reflect their current status and match the convention used by proposals in the swift-evolution repo.

Modifications

• Update the Status field of all accepted proposals which have been fully implemented to "Implemented".
• Indicate the Swift version in which those proposals were implemented.
• Add any missing links to Forum reviews or acceptance posts.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.