Breaking Change: protoc: require that reserved names are identifiers

[jhump]

This changes the current warnings into errors. In the previous PR (#10586), we downgraded these to warnings with the decision that they could be upgraded back to errors in a future major version.

Now that v24.0 is on the cusp of release, this seemed like a reasonable time to change it to error.

Resolves #6335 and #4558.

[jhump]

@mkruskal-google, is now an appropriate time to make this change?

[fowles]

We are planning some breaking changes for the January release, so I suspect the best time for this is immediately after the October release.

[jhump]

We are planning some breaking changes for the January release

@fowles, would that be a good time to address grammar changes, too (#13440)?

[zhangskz]

I believe the grammar changes described in #13440 isn't a breaking change and thus wouldn't need to wait until January to be released.

[zhangskz]

Perhaps this change could be added to the PROTOBUF_FUTURE_BEAKING_CHANGES queue for the next breaking release?

protobuf/src/google/protobuf/port_def.inc

Line 231 in 0681fe6

#ifdef PROTOBUF_FUTURE_BREAKING_CHANGES

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[mkruskal-google]

IIRC this is the grammar change we bundled into edition 2023, so closing this :)