Change the default branch of portable-simd to main

[Kobzol]

Related PR: rust-lang/portable-simd#549

[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/portable-simd':
      Rulesets:
          Creating 'main'
            Include Branches: ["refs/heads/main"]
            Required approvals: 0
          Deleting 'master'

[programmerjake]

lgtm as a project-portable-simd member, though idk what order we should be merging the master->main PRs in so I'll leave that part to someone else.