Fix Checkstyle configuration for nullability annotations

[sbrannen]

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.

Related Issues

• Use jspecify Nullable #35114
• Remove redundant value from illegalClasses for IllegalImportCheck #35195