textproc/UCD and deskutils/gucharmap: update to 15.1.0

Add comments about version sync requirement, and strengthen
deskutils/gucharmap BUILD_DEPENDS version specifier.

This also reverts commit 6ca002d89c860b63d4a925d30f1ed2a063aece91.

Reported by: olgeni
PR: 273796
Approved by: vishwin (gnome, maintainer deskutils/gucharmap)

		thierry (maintainer textproc/UCD)