[docs] Update docs to reflect latest WCAG 2.2

[oliviertassinari]

Steps to reproduce

Steps:

1. Open https://github.com/search?q=org%3Amui+%22WCAG+2.1%22&type=code
2. See the outdated references
3. Expand the search term, and find others

Current behavior

Material UI and MUI X docs still mention WCAG 2.1, even though it’s no longer the latest guideline as of October 2023. This creates the feeling that those components are “toys”; it’s not what “serious” looks like.

Expected behavior

Base UI does this correctly.

Context

From a legal standpoint:

• WCAG 2.1 level AA seems to be currently the target of all the important laws (United States ADA / Section 508, European Union EAA / EN 301 549, United Kingdom PSBAR / Equality Act).
• All those laws are moving to enforce WCAG 2.2 in 2026 or 2027, depending on the region.

WCAG 3.0 seems unlikely to be completed; not recommended until 2028+. So not enforced by laws before 2030. There is time.

I realized this from https://www.notion.so/mui-org/34fcbfe7b6608033bad4e0a2226ef554.

Your environment

npx @mui/envinfo

  Don't forget to mention which browser you used.
  Output from `npx @mui/envinfo` goes here.

Search keywords: -

[gitbasitmalik]

Hi @oliviertassinari, I'm a developer looking to make my first contribution to MUI. I've reviewed the search results for 'WCAG 2.1' and would be happy to help update these to 'WCAG 2.2' to ensure the docs reflect the latest standards.
Since this is currently 'waiting for maintainer,' is this a task you are ready to open up to the community, or are you handling this internally?