Add font sizes scale to Tailwind's configuration

What does this MR do and why?

This adds the supported font sizes to Tailwind's configuration.

This also migrates existing usages of Tailwind font-size utils to their new name, as per the newly introduced configuration options.

Some of Tailwind's font-size utils have been introduced before we could configure the proper sizing scale. Now that the scale is set, we need to migrate existing usages to the new names that correspond to the intended sizes.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

No visual change. Previously used utils have been migrated so that the same font sizes are being applied.

Before	After

How to set up and validate locally

An example would be the Edited <time> ago text in an issuable:

• Edit any issue.
• Scroll down to the end of the issue's description.
• Observe the Edited <time> ago text.

Related to #456640