Change the default value for `preloadDialectsInContext` for MlirOptMain

This option has been deprecated for 6 months, change the default setting for now before
future removal.

While clients can set the option to true for now, they should start
updating their passes to define the right `dependentDialects` in
preparation of the removal of this option. See the FAQ for more info:
https://mlir.llvm.org/getting_started/Faq/

Reviewed By: rriddle, jpienaar

Differential Revision: https://reviews.llvm.org/D99025