Hard deprecate Preprocessor_DOM

Hard deprecate Preprocessor_DOM

The Preprocessor_DOM implementation doesn't interact well with PHP memory
profiling, and has some limitations not present in the Preprocessor_Hash
implementation (see T216664).  There is no reason to keep around two
versions of the preprocessor: it just complicates on-going wikitext
feature development.

Hard deprecate use of Preprocessor_DOM, so we can remove the redundant
code in a future release.

Bug: T204945
Depends-On: Id38c9360e4d02b570996dbf7a660f964f02f1a2c
Change-Id: Ica5d1ad5b1e677542962fc36d582a793f941155e