go.mod: switch to gogoproto fork

[RaduBerinde]

Use github.com/cockroachdb/gogoproto instead of
github.com/gogo/protobuf. The former already has a new omitempty
option that can clean up some existing hacks, and we want to make more
improvements to the generated code.

Fixes #134805
Release note: None

[cockroach-teamcity]

This change is 

[tbg]

Is there a world here in which we don't need a version in the replace directive? The moment you pin a version, you basically bypass version requirement resolution. I would love to get rid of the replace line for protobuf, which I seem to remember is causing us a lot of trouble and this is also somehow related to gogoproto (it isn't compatible with v2 protobuf?)
I know this isn't exactly in your critical path, but I think we have a little dependency hell here. Forking gogoproto can probably help here, but if we're pinning a version it could also go the other way. Could we get a sense of how deep the pit is? I'm happy to team up on that.

[tbg]

Nevermind, we have to provide a version on the right: https://go.dev/ref/mod#go-mod-file-replace
But it seems possible to specify a version on the left, which could let us opt out of letting dependencies that use gogoproto see the replacement. I don't know if that is helpful at all but it would reduce the amount of lying.

Re: protobuf, we have require github.com/gogo/protobuf v1.3.2 but then replace google.golang.org/protobuf => google.golang.org/protobuf v1.29.1, so we're straight-up lying about which version of googleproto we see. And I think we need that because (something something gogoproto not compatible with newer googleproto). That would be good to lift if we can.

I'm poking at it a bit in #134941.

[RaduBerinde]

But it seems possible to specify a version on the left, which could let us opt out of letting dependencies that use gogoproto see the replacement. I don't know if that is helpful at all but it would reduce the amount of lying.

How would that work? Our binary must contain exactly one version of the library..

[RaduBerinde]

TFTR!

bors r+

[craig]

Build failed (retrying...):

• check_generated_code

[RaduBerinde]

bors r-

[craig]

Canceled.

[RaduBerinde]

bors r+

[craig]

Build succeeded:

• examples_orms
• macos_amd64_build
• acceptance
• check_generated_code
• linux_amd64_build
• docker_image_amd64
• linux_amd64_fips_build
• windows_build
• macos_arm64_build
• linux_arm64_build
• local_roachtest
• unit_tests
• lint
• local_roachtest_fips