Use URW fonts instead of TeX Gyre as the default

These fonts are typographically preferable and include support for Greek and Cyrillic as well as Latin (Greek support had defaulted to the unusable Gyre Greek instead). The Gyre fonts are kept as a fallback.

This changes the URW 2.0 fonts to be the default (see issue #6002 (closed)). These fonts are already required/included in our distribution, so there is no change needed there. The changelog will need to be updated, and the text file in documentation will need to be changed.

In brief, for the reasons specified in #6002 (closed), all the TeX Gyre fonts (similar issues exist in all of them, not just Schola) are set only to be backups to the URW 2.0 fonts.