Support protobuf v21.0 rc1

[Katee]

I've also tested this using v21.4 (the latest release) but did not test any releases between those two.

I tested using:

$ npm install
$ bazel build @com_google_protobuf//:protoc
$ PROTOC="$(realpath bazel-bin/external/com_google_protobuf/protoc)" PROTOC_INC="$(realpath bazel-protobuf-javascript/external/com_google_protobuf/src/)" npm test

It may also make sense to update the version of protobuf-javascript here to match the version of protobuf.

[dibenede]

I've also tested this using v21.4 (the latest release) but did not test any releases between those two.

What's the motivation for using v21.0 rc1 specifically?

It may also make sense to update the version of protobuf-javascript here to match the version of protobuf.

The current plan is to just continue the versioning we started with.

[Katee]

What's the motivation for using v21.0 rc1 specifically?

v21.0 rc1 is the first release of protobuf after the javascript folder was removed: https://github.com/protocolbuffers/protobuf/releases/tag/v21.0-rc1 v21.0 could also make sense since that's the first non-candidate version. v21.4 could also makes sense since it is the most recent version.

The current plan is to just continue the versioning we started with.

Sure, you certainly don't need to match the protobuf version.