Fix: Add fallback for clipboard.writeText

[heybran]

Description

Copy button near field name previously was using navigator.clipboard.writeText, but I just found out that the API is only available for secure context: https://developer.mozilla.org/en-US/docs/Web/API/Clipboard/writeText

So if developers are working locally on insecure context, copy button will fail. I believe I was working with https when I was adding this feature back then, so I didn't really catch the bug.

So I'm sending a fix PR to add a fallback to insecure context or old browsers.

Related GitHub issue(s)

#7237

Testing instructions

1. Spin up a local site without https
2. Find the copy button
3. Click the button, now the field name will be copied.

Screenshots / screencast

Changelog text for these changes

Bug: Copy button now works correctly in secure context. #7237 (@heybran)

PR checklist

• I have tested my own code to confirm it works as I intended.
• My code follows the WordPress Coding Standards.
• My code follows the WordPress Inline Documentation Standards.
• My code includes automated tests for PHP and/or JS (if applicable).