[2.1] Replaces uses of deprecated constant SID with calls to session_id()

[Sesquipedalian]

Fixes #8365

[sbulen]

I think this is an issue... I believe the constant SID includes more than just session_id...

From the PHP manual:
SID (string)
Constant containing either the session name and session ID in the form of "name=ID" or empty string if session ID was set in an appropriate session cookie. This is the same id as the one returned by session_id().

In other words, I think the replacement for the constant SID must look something like: "PHPSESSID=yadayadayada". session_id is only the yadayadayada part....

Note also that the "PHPSESSID" part can also be renamed in session.name as well, e.g., by the site admin.

[Sesquipedalian]

For now, I have reverted these changes and added a stop-gap solution for SMF 2.1 in #8388