slack-vitess-r15.0.5: backport v20 vtgate syscall/topo-call optimizations

[timvaillancourt]

Description

This PR backports several v20 PRs that address runtime crashes of vtgate due to too many (10k+) concurrent syscalls

PRs:

1. Update MySQL apt package and GPG signature vitessio/vitess#14785 (already merged)
2. Load --grpc_auth_static_client_creds file once vitessio/vitess#15030
3. Limit concurrent creation of healthcheck gRPC connections vitessio/vitess#15053
4. Filter by keyspace earlier in tabletgateways WaitForTablets(...) vitessio/vitess#15347

Related Issue(s)

1. Feature Request: cache static auth creds in go/vt/grpcclient vitessio/vitess#15031

PRs above

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @mattlord to sign the Salesforce Inc. Contributor License Agreement.