Unlock google.golang.org/grpc

[TBBle]

This was originally locked in #1000 based on containerd's resolution for containerd/ttrpc#62. However, the lock on google.golang.org/grpc was dropped in containerd/containerd#5613 while upgrading libraries we don't actually use, so it was probably never applicable for us, while the lock on google.golang.org/genproto was kept.

See also containerd/ttrpc#89, which intended to document that only google.golang.org/genproto needs to be locked. However, this PR was dropped in favour of containerd/ttrpc#92, dropping gogo/protobuf entirely (somehow).

Draft because:

• Needs to be updated to align with the approach in containerd instead, see Unlock google.golang.org/grpc #1177 (comment)
• This was an "in-passing" inspiration, and I haven't tested it in any way except the CI test run. I suspect someone with more expertise is already taking care of this in concert with the containerd changes.

[TBBle]

Note to self: Protobuf implementation regeneration command is in #1175, e.g., 02cfac9.

[TBBle]

For future work, #1302 has pointed out that containerd/ttrpc#92 has an overarching issue at containerd/containerd#6564 which might be clearer about how to migrate away from github.com/gogo/protobuf.

[TBBle]

Rebased onto master, incorporating #1302 so protobuf regeneration is now documented and verified in GitHub Actions.

containerd/containerd#6564 has been closed by merge of containerd/containerd#6841 (and also containerd/containerd#6867 is a relevant follow-up) which still carries the replaces for genproto, but containerd/containerd#5208 (comment) notes that these may be able to be removed now.