Allow compilation with Plan B deprecation enabled

This change introduces mechanisms to allow the codebase to compile
even when deprecate_plan_b = true is set in GN args:

1.  A new GN config suppress_plan_b_deprecation is added to suppress
    deprecated-declarations warnings.
2.  The rtc_test template is updated to automatically apply this
    suppression, allowing unit tests to continue calling Plan B APIs.
3.  RTC_ALLOW_PLAN_B_DEPRECATION_BEGIN/END macros are added to
    plan_b_only.h to locally suppress warnings in production glue code.
4.  Existing call sites in PeerConnection and SdpOfferAnswerHandler
    are wrapped with these macros.
5.  Updated g3doc/plan_b_removal.md with the new macros and usage.

Bug: webrtc:467984837
Change-Id: I42bc995b7825e0ae96db8919dd31d2c44551f430
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/442362
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46682}