release-21.1: go.mod: switch google cloud to fork of v0.45.1

[adityamaru]

This change switches the release-21.1 branch to point to a
fork of v0.45.1 of the google cloud SDK that includes
googleapis/google-cloud-go#4226.

The above change provides better retry on GOAWAY errors which
have become an increasingly common occurrence in our roachtests.

Release note (general change): Switches the release-21.1 branch
to point to a fork of the google cloud SDK at version 0.45.1
with an upstream bug fix
googleapis/google-cloud-go#4226.

[cockroach-teamcity]

This change is 

[adityamaru]

For posterity the steps I followed were:

• Created a branch on our fork off of v0.45.1 - https://github.com/cockroachdb/google-cloud-go/tree/goaway-retry
• Created a branch of release-21.1
  • Edited go.mod to point to the branch specified above.
  • go mod tidy, make vendor_rebuild
  • Committed the changes in vendor and pushed to https://github.com/cockroachdb/vendored/commits/google-sdk-goaway-retry-v21.1

*one thing I'm not sure about is why the vendor diff is not showing the changes to the test files. egs: storage/reader_test

[pbardea]

*one thing I'm not sure about is why the vendor diff is not showing the changes to the test files. egs: storage/reader_test

It looks like the version that we're patching on top of didn't have any of these test files (see
https://github.com/cockroachdb/vendored/tree/62f7cec4ff212ca2a8e0edef07f68fc47b3b8ae0/cloud.google.com/go/storage) so I think the patch wasn't applied (since there's no file to apply to).

The change LGTM; the change looks fairly self-contained and applies the same to this version as it did to the current one.