Move std-dev-guide from master to main

[clarfonthey]

Associated PR for that repo: rust-lang/std-dev-guide#81

Note: that PR also modifies a few unrelated links to renamed branches from other repos too, in addition to just changing its master branch to main.

[github-actions]

Dry-run check results

[WARN  rust_team::sync] sync-team is running in dry mode, no changes will be applied.
[INFO  rust_team::sync] synchronizing crates-io
[INFO  rust_team::sync] synchronizing github
[INFO  rust_team::sync] 💻 Repo Diffs:
    📝 Editing repo 'rust-lang/std-dev-guide':
      Rulesets:
          Creating 'main'
            Include Branches: ["refs/heads/main"]
            Require branches to be up to date: true
            Required approvals: 1
            Required status checks: book-test (integration_id: 15368)
          Deleting 'master'