multimedia/mythtv{,-frontend}: Revert placebo version updates

In r440321 and r440322 PORTVERSION was bumped to 0.28.7/0.28.1, but
GH_TAGNAME was not updated. Commit ad97d24 is tagged as v0.27.5 [1].

Reset version back to 0.27.5 and bump PORTEPOCH.

[1] https://github.com/MythTV/mythtv/releases/tag/v0.27.5

PR: 222219
Reported by: robbak@gmail.com
Pointy hat: miwi