`PlaceholderUnificationVisitor` using javac 21 API

In f093681 the `PlaceholderUnificationVisitor` class was modified to call `CaseTree.getGuard()`:

https://github.com/google/error-prone/blob/cc12b63b439df353672c89d08acba3baa4c8b43f/core/src/main/java/com/google/errorprone/refaster/PlaceholderUnificationVisitor.java#L665

But, this API was only introduced in JDK 21:

https://github.com/openjdk/jdk/blob/9477c705c0bd5ce2d445abb5ca44d46656fc315f/src/jdk.compiler/share/classes/com/sun/source/tree/CaseTree.java#L78-L84

So I think this code will fail if it is run on JDK 17 (which Error Prone still supports). Maybe this wasn't detected on CI due to missing test coverage?