make prevent-update default

[marcoieni]

Discussed in https://github.com/rust-lang/team/pull/2327/changes#r2986773168

[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/bors':
      Rulesets:
          Updating 'main'
            Restrict updates: true
    📝 Editing repo 'rust-lang/cargo':
      Rulesets:
          Updating 'master'
            Restrict updates: true
          Updating 'rust-1.*'
            Restrict updates: true
    📝 Editing repo 'rust-lang/crates.io':
      Rulesets:
          Updating 'main'
            Bypass Actors: [] => [RulesetBypassActor { actor_id: 2201425, actor_type: Integration, bypass_mode: Always }]
            Restrict updates: true
    📝 Editing repo 'rust-lang/rustfmt':
      Rulesets:
          Updating 'main'
            Restrict updates: true
          Updating 'libsyntax'
            Restrict updates: true
          Updating 'rust-1.*'
            Restrict updates: true

[marcoieni]

closed in favor of #2346