Summary
-------
Deprecate the `ParallelRefProcEnabled` flag in JDK 26 and obsolete it in JDK 27, then remove in JDK 28.

Problem
-------

`ParallelRefProcEnabled` incurs some implementation complexity in reference processing. There is no benefit in disabling this flag as automatic dynamic thread number selection will automatically determine the best number of threads to use per type of references, from all available parallel threads down to using only a single thread.

This flag is used only by Parallel and G1, and both have it enabled by default via:

```
  if (FLAG_IS_DEFAULT(ParallelRefProcEnabled) && ParallelGCThreads > 1) {
    FLAG_SET_DEFAULT(ParallelRefProcEnabled, true);
  }
```

Solution
--------

Deprecate the `ParallelRefProcEnabled` flag in JDK 26 and obsolete it in JDK 27, then remove in JDK 28.

Specification
-------------

```
   product(bool, ParallelRefProcEnabled, false,                              \
-          "Enable parallel reference processing whenever possible")         \
+          "(Deprecated) Enable parallel reference processing "              \
+          "whenever possible")                                              \
```