add migration rule to change preload to hx-preload in upgrade checker

[AshwinSundar]

Description

Please describe what changes you made, and why you feel they are necessary. Make sure to include
code examples, where applicable.

Htmx 2 docs recommend calling the extension with the preload attribute ( https://htmx.org/extensions/preload/),, "‌") while in htmx 4 you need to call the attribute hx-preload. This PR adds a migration rule to the Upgrade Checker to warn the user of this change.

Corresponding issue:

Testing

Please explain how you tested this change manually, and, if applicable, what new tests you added. If
you're making a change to just the website, you can omit this section.

I had this issue in a codebase which uses HTMX, and ran ./upgrade-check.js against my repo to make sure it found any usage of preload instead of hx-preload.

I ran npm test locally to make sure all 87 test files passed.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded