Obsolete "Pragma" HTTP header

[hannob]

There are a couple of places in the code that set the "Pragma:" http header:

index.php
plugins/serendipity_event_gravatar/serendipity_event_gravatar.php
serendipity.css.php

According to MDN and current HTTP caching specs (RFC 9111), it is obsolete and discouraged:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Pragma
https://httpwg.org/specs/rfc9111.html#field.pragma

It looks like it was only part of HTTP/1.0 and got replaced by Cache-Control a loooong time ago. I believe we can simply remove that and it'll have no consequences.

[onli]

Hm. And also, it was only meant for Requests, not responses.

There is one counter argument, following https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Pragma: The browser table at the bottom shows full support of the header in all browsers. While it is likely the other headers we set make the pragma header redundant - it is followed by an Expires header everywhere but the gravatar plugin, but that plugin does set a Cache-Control header - there is a very small chance that removing the header might cause browsers to do something different now.

I'm still okay with removing it and have created a PR at #950