Make lintrunner work with new main branch (#99466)

We've renamed the `master` branch to `main`. Lintrunner should check for a merge base from this new branch now

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### <samp>🤖 Generated by Copilot at 9743d70</samp>

Updated the linter configuration to reflect the new default branch name. Changed `merge_base_with` from `origin/master` to `origin/main` in `.lintrunner.toml`.

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copilot:poem
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### <samp>🤖 Generated by Copilot at 9743d70</samp>

> _`merge_base_with` changed_
> _`origin/main` is the new_
> _branch name for pytorch_

Pull Request resolved: https://github.com/pytorch/pytorch/pull/99466
Approved by: https://github.com/kit1980, https://github.com/malfet
diff --git a/.lintrunner.toml b/.lintrunner.toml
index 759a751..9abf1b9 100644
--- a/.lintrunner.toml
+++ b/.lintrunner.toml
@@ -1,4 +1,4 @@
-merge_base_with = "origin/master"
+merge_base_with = "origin/main"
 
 [[linter]]
 code = 'FLAKE8'