OFL FAQ URL in OFL.txt is not checked

[n8willis]

Related to #4843 , the URL of the OFL FAQ that exists in the OFL.txt file in the upstream font repo seems like it does not get checked for current-ness.

Observed behaviour

An OFL.txt file that has the old SIL .org address for the OFL FAQ does not raise any warning.

Expected behaviour

If the old URL must be replaced by the new one, then a check result similar to what Fontbakery produces for an incorrect URL in the name table would be expected.

Resources and steps needed to reproduce

Visible at google/fonts#8208