[CSS] T0.1 Formalize the browser floor #17709

Part of #17707.

Our theme styles already depend on container style queries, which need Firefox 151+ and Safari 18+. Angular's default browser targets are lower (Chrome/Edge/Firefox 119+, Safari 17), so on those older versions the design-system rules don't apply today. This task documents the floor we actually have, so the rest of #17669 can rely on it.

What to do

1. Check the Firefox ESR. Confirm the current Firefox ESR version. If an ESR below 151 is still in its support window, raise it here before going further.
2. Add the library browserslist at projects/igniteui-angular/.browserslistrc:

   Chrome >= 131
   Edge >= 131
   Firefox >= 151
   Safari >= 18
   iOS >= 18
   

3. Document it. Add a "Browser support" section to the README and mention it in the release notes.
4. Compare with Web Components. Check which floor Ignite UI for Web Components uses, and note any difference here.

Done when

• The browserslist and the README section are merged.
• The ESR check result is recorded in this issue.