Prepare for v1.0.3

[jhump]

The following is a draft of the release notes:

---

What's Changed

Bugfixes

• Update test cases for consistency, backfill missing cancel tests by @jhump in Update test cases for consistency, backfill missing cancel tests #874
• Fix reference server to verify that client implementations properly enforce timeouts by @jhump in Require that client implementations properly enforce timeouts #877
• Use strict codecs in reference client and server to produce better error messages in the face of certain codec/format errors by @jhump in Use strict codecs in reference client and server #878
• Fix the Connect GET test suite configuration so those tests are only run when a client/server supports GET by @jhump in Update the Connect GET test suite configuration so it's only run when a client/server supports GET #883
• Add missing gRPC tests for trailers-only responses by @jhump in Add gRPC tests for trailers-only #904

Other changes

• Update to use Go to 1.21 by @jhump in Update Go to 1.21 and quic-go to 0.43.0 (which now requires Go 1.21) #866
• Deprecate the textpb codec by @jhump in Deprecate the textpb codec #875

Full Changelog: v1.0.2...v1.0.3