Prepare for v1.17.0 release

[jhump]

Below are draft release notes for this release. I decided to make it a minor version instead of a point release because:

• Supporting editions source files (Enable protoc-gen-connect-go usage with editions #754) could be seen as an enhancement (even though it makes no real changes to the code and doesn't add any features or new API -- it just toggles something in the code gen response to allow it).
• Updating to require Go 1.21 (Require go 1.21, test with go 1.23 #770) seems like something worthy of a minor version instead of a point release.

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v1.17.0

What's Changed

Enhancements

• Enable protoc-gen-connect-go usage with Protobuf source files that use Editions by @jchadwick-buf in Enable protoc-gen-connect-go usage with editions #754

Bugfixes

• Prevent incorrect propagation of protocol-specific metadata by @emcfarlane in Restrict handler metadata headers #748
• Fix error message about unexpected content-type by @jhump in Fix error message about content-types #775
• Calls should return "unavailable" instead of "unimplemented" or "unknown" when the transport fails to produce an HTTP response and returns io.EOF errors by @jhump in Connect unary should return unavailable instead of unimplemented for io.EOF errors from the transport #776

Other changes

• Now requires Go 1.21 by @jhump in Require go 1.21, test with go 1.23 #770

New Contributors

• @perezd made their first contribution in Adding OpenSSF badge #739

Full Changelog: v1.16.2...v1.17.0