Merge latest main before running Percy

[jmuzina]

Done

• Modifies the Percy "prepare" workflow to check out a merge commit of main and a PR, rather than the PR head commit.
  • Reduces the need for us to rebase/merge main into each PR before running Percy.
  • Manual rebasing/merging will still need to happen if conflicts are present between the PR and main - Percy will not be triggered if merge conflicts are detected, since GH doesn't run workflows on PRs with merge conflicts.
• Removes the repository parameter from the Percy prepare job as it is no longer needed.
• Adds some more helpful input parameter documentation for the Percy prepare job.

Context:

Merging main into the checked out commit is default behavior for actions/checkout, but it is documented poorly (see issue). Our current code is forcing actions/checkout to check out the head of the PR (ignoring main) - so all we need to do in order to merge main into the checked out code is get rid of repository and ref inputs to actions/checkout.

Fixes #5282, WD-13965

QA

• Review test PR
  • Download the percy artifact uploaded by the prepare job
  • Open scss/_vanilla.scss and verify line 4 reads // This was added by the PR Test.
  • Verify that this modification to _vanilla.scss is not in the main branch of my fork, and was added by the test PR.
  • Open scss/_base.scss and verify line 2 reads // This was added by the main branch.
  • Verify that this modification to _base.scss is only in the main branch of my fork, and is not present in my PR's source.

Check if PR is ready for release

If this PR contains Vanilla SCSS code changes, it should contain the following changes to make sure it's ready for the release:

• PR should have one of the following labels to automatically categorise it in release notes:
  • Feature 🎁, Breaking Change 💣, Bug 🐛, Documentation 📝, Maintenance 🔨.
• Vanilla version in package.json should be updated relative to the most recent release, following semver convention:
  • if CSS class names are not changed it can be bugfix release (x.x.X)
  • if CSS class names are changed/added/removed it should be minor version (x.X.0)
  • see the wiki for more details
• Any changes to component class names (new patterns, variants, removed or added features) should be listed on the what's new page.

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