Tweak - Meta key field readonly

[riteelama]

All Submissions:

• Have you followed the Everest Forms Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This PR makes the meta key field readonly.

Closes # .

How to test the changes in this Pull Request:

1. Go to All Forms and edit the form.
2. Click on any field and check whether the meta key field is readonly or not.

Types of changes:

• Bug fix (non-breaking change which fixes an issue)
• Enhancement (modification of the currently available functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully ran tests with your changes locally?
• Have you updated the documentation accordingly?

Changelog entry

Tweak - Meta key field readonly.

[deepench]

LGTM 👍