[sdk-platform-java] Stop using javax.annotations.Nullable from com.google.code.findbugs:jsr305

A child issue of googleapis/google-cloud-java#12487. We use the following annotations from Findbugs's jsr305.

- javax.annotation.Nonnull (RetentionPolicy.RUNTIME) This comes from com.google.code.findbugs:jsr305:jar:3.0.2
- javax.annotation.Nullable (RetentionPolicy.RUNTIME) This comes from com.google.code.findbugs:jsr305:jar:3.0.2

Related:

- Guava's https://github.com/google/guava/issues/2960 (still open)
- "_F. JAVA TECHNOLOGY RESTRICTIONS. You may not create, modify, or change the behavior of, or authorize your licensees to create, modify, or change the behavior of, classes, interfaces, or subpackages that are in any way identified as "java", "javax", "sun", “oracle” or similar convention as specified by Oracle in any naming convention designation._" Therefore, the Findbugs project, which has been discontinued, cannot release `javax.annotation.Nonnull` or `javax.annotation.Nullable` any more. We should stop using the annotations.
- Guava uses checkerframework's Nullable: https://github.com/google/guava/blob/master/guava/src/com/google/common/cache/LocalCache.java#L91
- Guava still uses `javax.annotation.Nonnull` https://github.com/google/guava/blob/b567b26e2bd6ea679b8cf69711cc8a41ecf219c4/guava/src/com/google/common/cache/ElementTypesAreNonnullByDefault.java#L28
- Findbugs project has been forked as the Spotbugs project "FindBugs is now SpotBugs" https://github.com/findbugsproject/findbugs.