docs: Simplify Apache httpd proxy documentation

[igalic]

Description

since 2.4.47 (released April 22nd 2021), Apache httpd can ProxyPass to websockets on the same URL, without mod_rewrite (and, without mod_proxy_wstunnel).

Checklist

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have made any necessary changes to documentation.

[tsmethurst]

Thanks for doing this! ^_^