cppgc: Remove experimental notice

Bug: chromium:1056170
Change-Id: I71375a2ac8f4ac51de8b6c1899f7f146e4f107ec
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3099949
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76346}

include/cppgc/README.md

 # C++ Garbage Collection
 
 This directory provides an open-source garbage collection library for C++.
-
-The library is under construction, meaning that *all APIs in this directory are incomplete and considered unstable and should not be used*.
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