Add missing @Nullable to setters of Nullable Fields

There are setters and builder methods that initialize members that are
`@Nullable` but do not accept `@Nullable` parameters.

For example:

```
private @Nullable Object foo;

public void setFoo(Object foo) {
    this.foo = foo;
}
```

It is an unnecessary restriction that the parameter is unable to be null
since the field can be null.

This commit fixes these inconsistencies.

Closes gh-18618

core/src/main/java/org/springframework/security/authorization/RequiredFactor.java

@@ -122,7 +122,7 @@ public final class RequiredFactor {
 		 * @param authority the authority.
 		 * @return the builder.
 		 */
-		public Builder authority(String authority) {
+		public Builder authority(@Nullable String authority) {
 			this.authority = authority;
 			return this;
 		}
@@ -205,7 +205,7 @@ public final class RequiredFactor {
 		 * @param validDuration the {@link Duration}.
 		 * @return
 		 */
-		public Builder validDuration(Duration validDuration) {
+		public Builder validDuration(@Nullable Duration validDuration) {
 			this.validDuration = validDuration;
 			return this;
 		}