docs: Editorial pass on howto/http2.xml

docs: Editorial pass on howto/http2.xml

- Grammar and spelling fixes throughout (fixes bz#70115)
- Wrap overlong lines to match document style
- Replace dead browser extension links with current equivalents
- Update caniuse link to https
- Simplify browser support list (all modern browsers support HTTP/2)
- Note Server Push deprecation per RFC 9113; point to Early Hints
- Note Accept-Push-Policy draft expired and was never adopted

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1935470 13f79535-47bb-0310-9956-ffa450edef68