Spring Fix Checkstyle configuration for nullability annotations

Overview

While assessing #35195, I noticed that there are a few issues with our Checkstyle configuration regarding nullability annotations.

^(?!org\.jspecify|\.annotations).*(NonNull|Nullable)$ contains a |.
^(?!org\.jspecify|\.annotations).*(NonNull|Nullable)$ matches against NonNull but not against Nonnull, and therefore incorrectly permits usage of javax.annotation.Nonnull.
Some of the Checkstyle suppressions no longer apply.