Require linear history in lang-team repo

[tmandry]

We don't use bors, so all three options to merge a pull request appear. Historically PRs have used merge commits, but this was not enforced.

There's not a need for merge commits in a repo as simple as lang-team, and there's not an option to require them for consistency either. For simplicity, let's disallow future merge commits and keep a linear history.

[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/lang-team':
      Rulesets:
          Creating 'main'
            Include Branches: ["refs/heads/main"]
            Require linear history: true
            Required approvals: 1

[rustbot]

☔ The latest upstream changes (possibly #2550) made this pull request unmergeable. Please resolve the merge conflicts.