Fix: Custom font size taking over fit text

[jorgefilipecosta]

Currently we have an issue where if a paragraph/heading had a custom font size and then was converted to the stretchy variation. The custom font size on the front end is applied instead of fit text.
It was fixed in Gutenberg at ​https://github.com/WordPress/gutenberg/pull/73241, and this ticket tracks the fix in WordPress core.

Backports ​https://github.com/WordPress/gutenberg/pull/73241 into the core.

Fixes an issue where if a paragraph/heading had a custom font size and then was conververted to the stretchy variation. The custom font size on the front end was applied instead of fit text.

cc: @mrwweb

Ticket: https://core.trac.wordpress.org/ticket/64254

## Testing
Insert Paragraph block
Type some text in it
Set a non-default font size for the Paragraph block
Switch the Paragraph to the fit text Paragraph variation
The front-end of the site should not use the font size setting selected in Step 3 and should instead show fit text working normally as in the editor.

[jorgefilipecosta]

In 61246:

Fix: Custom font size taking over fit text.

Fixes an issue where if a paragraph/heading had a custom font size and then was conververted to the stretchy variation. The custom font size on the front end was applied instead of fit text.

Developed in: ​https://github.com/WordPress/wordpress-develop/pull/10517

Props jorgefilipecosta, mcsf, mrwweb.
Fixes #64254.

[jorgefilipecosta]

Reopening for backport.

[wildworks]

Reopening for backport.

Approved ✅

[jorgefilipecosta]

In 61251:

Fix: Custom font size taking over fit text.

Fixes an issue where if a paragraph/heading had a custom font size and then was conververted to the stretchy variation. The custom font size on the front end was applied instead of fit text.

Developed in: ​https://github.com/WordPress/wordpress-develop/pull/10517.

Props jorgefilipecosta, mcsf, mrwweb, t-hamano.
Fixes #64254.