[0.76] Bump Metro to 0.81.0-alpha

[robhogan]

Summary:

Bump's Metro in 0.76-stable to 0.81.0-alpha.0, this is the Metro release targeting 0.76 and will stabilise to ^0.81.0 before the final RC.

Effectively picks #46398

Changelog:

[General][Changed] - Bump Metro packages to 0.81.0-alpha

Test Plan:

• Smoke test RNTester, correct Metro version is reported, app bundles, Fast Refresh and DevTools work
• Same version as used on main and internally

[react-native-bot]

	Warnings
⚠️	🔒 package.json - Changes were made to package.json. This will require a manual import by a Facebook employee.

Generated by 🚫 dangerJS against 1110bab