fix(mergify): remove auto-update rule that adds merge commits

[LalatenduMohanty]

The update action defaulted to merge, causing Mergify to add "Merge branch 'main' into " commits to PRs instead of rebasing them.

Closes: #1069

• PR follows CONTRIBUTING.md guidelines

[coderabbitai]

📝 Walkthrough

Walkthrough

This change modifies the .mergify.yml configuration file to specify method: rebase for the "Auto-update approved PRs that are behind" rule. Previously, the update action had no explicit method specified, defaulting to merge behavior. The addition explicitly configures the Mergify bot to rebase PRs onto the main branch rather than creating merge commits.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

🚥 Pre-merge checks | ✅ 3 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Title check	⚠️ Warning	The title states 'remove auto-update rule' but the actual change adds method: rebase to the existing rule—it modifies rather than removes it.	Update title to reflect the actual change: 'fix(mergify): use rebase method for auto-update rule' or similar.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description check	✅ Passed	Description explains the problem (default merge behavior) and references the linked issue, relating directly to the changeset.
Linked Issues check	✅ Passed	The PR successfully implements the fix from issue #1069: adding method: rebase to the update action prevents merge commits.
Out of Scope Changes check	✅ Passed	The changeset is focused solely on the Mergify configuration change required by issue #1069 with no extraneous modifications.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

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[rd4398]

LGTM!