Remove redundant #available(iOS 12, *) checks - #19511

They always return true since the deployment version is higher than iOS 12.

I was looking at #available statements as part of #19496 and realized we still has some of these laying around.

There are even #available(iOS 11, *) and below showing up in the project, but they are all in the Pods or in SnapshotHelper.swift, which is code generated.

Regression Notes

1. Potential unintended areas of impact – N.A.
2. What I did to test those areas of impact (or what existing automated tests I relied on) – N.A.
3. What automated tests I added (or what prevented me from doing so) – N.A.

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes. N.A.
• I have considered adding accessibility improvements for my changes. N.A.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary. N.A.