slack-19.0: backport v20 PRs, set slack-15.0 as previous release

[timvaillancourt]

Description

This PR contains the typical hacks for a Slack release such as:

1. CI runner updates to slackhq self-hosted
2. Delete skip logic in tests (always run full CI)
3. Private repo git-config for https://github.com/slackhq/vitess-addons
4. CODEOWNERS setup for @slackhq/vitess-approvers
5. Previous release set to https://github.com/slackhq/vitess/tree/slack-15.0

Also all v20 backports we require we cherry picked

Backports

1. Load --grpc_auth_static_client_creds file once vitessio/vitess#15030
2. Limit concurrent creation of healthcheck gRPC connections vitessio/vitess#15053
3. Filter by keyspace earlier in tabletgateways WaitForTablets(...) vitessio/vitess#15347
4. Make Durabler interface methods public vitessio/vitess#15548

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

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