Deprecate IPUtils in favour of symfony/http-foundation

[GuySartorelli]

The IPUtils class is a direct copy from the class of the same name in symfony/http-foundation.
We should deprecate our class, and just include symfony/http-foundation as a direct dependency, directing people to use it instead.

Reasoning

1. Currently some of our methods in that class are out of date, and don't have improvements Symfony has made. Using their package directly obviously keeps it up to date.
2. We're left maintaining code that someone else is already maintaining. That's just silly.
3. In general, there's a desire currently to offload more of our code to third-party libraries which are more mature and better situated to maintain that code. This aligns with that desire.

Acceptance criteria

• Remove the old code and replace it with symfony dependency
• Deprecate any code as necessary in CMS 5
• If we can simply swap it out and no unit tests break, then do the work directly in CMS 5 instead. Otherwise if we need to update the code that calls these methods, then target CMS 6

CMS 5 PRs

• DEP Use symfony for IPUtils #11344
• API Use symfony for IPUtils silverstripe-session-manager#210
• DOC Deprecate IPUtils developer-docs#561

CMS 6 PRs

• API Remove IPUtils #11346

[GuySartorelli]

@emteknetnz Please create a PR for the 5.3 changelog that says IPUtils has been deprecated, and to use the symfony class instead.

[GuySartorelli]

CMS 5 PRs merged. Reassigning to Steve for CMS 6 PRs

[GuySartorelli]

PRs merged