ThreadSafe is not a valid checker name

In v2.36.0, seems to have an implementation in [`ThreadSafeChecker.java`](https://github.com/google/error-prone/blob/v2.36.0/core/src/main/java/com/google/errorprone/bugpatterns/threadsafety/ThreadSafeChecker.java) by the advertised name, but it cannot be activated by either name:

```
$ ./mvnw clean test-compile -DepFlags='-XepDisableAllChecks -Xep:ThreadSafe:WARN'
[INFO] Scanning for projects...
...
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.12.1:compile (default-compile) on project example: Fatal error compiling: ThreadSafe is not a valid checker name -> [Help 1]
```

I think this is because it is not named in `BuiltInCheckerSuppliers.java` (and never has been).