Upgrade tailwind to 3.x

[amazingphilippe]

We could make the best use of tailwind if we upgrade it. As new CSS features are adopted by browsers, older versions of tailwind won't support them.

Newer versions of tailwind also add a ton of useful features to help us build stuff.

They have upgrade guides that we can use

1. From 1 to 2: https://v2.tailwindcss.com/docs/upgrading-to-v2
2. From 2 to 3: https://tailwindcss.com/docs/upgrade-guide

We could also take the opportunity to clean up our config and really think about some legacy gov.uk code like:

1. The base font size of 10px (instead of 16px)
2. The typographic scale
3. Breakpoints
4. Colours, like focus colour

[yaelberger-commits]

Test on admin with a PR and see what happens, a Bug Bash, see what breaks and some cleanup

[yaelberger-commits]

Hey team! Please add your planning poker estimate with Zenhub @amazingphilippe @andrewleith @jzbahrai @whabanks

[andrewleith]

Related PR: cds-snc/notification-admin#1821

[amazingphilippe]

Included tailwind 3 upgrade as part of cds-snc/notification-admin#1821. Ready for review

[YedidaZalik]

Phil was cleaning up work on main navigation yesterday and trying to fix
PR linked at bottom for someone to check

[andrewleith]

• @andrewleith to review