Change lang-team default branch to main

Only CI relevant seems to be GH pages environment.

The deploy workflow already hedges against `master` vs `main`, cf.
<https://github.com/rust-lang/lang-team/blob/11beedc287edaacb33d77fa18340c0ac097e5ece/.github/workflows/deploy.yml#L35>.

Author: jieyouxu

repos/rust-lang/lang-team.toml

@@ -12,5 +12,5 @@ types = "maintain"
 release = "maintain"
 
 [environments.github-pages]
-branches = ["gh-pages", "master"]
+branches = ["gh-pages", "main"]