mandoc: Bump version string to 1.14.6s20250613

The mandoc compiler is still at version 1.14.6, but we have imported
several snapshots since the version was bumped. Append a snapshot date
string to the version to reflect this.

Requested by: wosch
Discussed with: bapt

(cherry picked from commit af2949cd29ef453a63a6345de396be866138b051)