build(fdroid): drop unused foojay toolchain resolver; release v2.7.4

F-Droid's official build scanner rejects the foojay-resolver-convention
plugin because it can fetch a JDK over the network during the offline,
reproducible build. The plugin was inert here (no toolchain block ever
invoked it), so removing it leaves the app functionally identical to
2.7.3 and still reproducible byte-for-byte from source.

Bumps versionName to 2.7.4 (the release trigger) and retargets the
official fdroiddata recipe draft at v2.7.4, plus two recipe corrections
the fdroiddata CI caught on the v2.7.3 submission (MR !40967):
- Categories: 'Time' (retired in the current taxonomy) -> 'Calendar & Agenda'
- AutoUpdateMode: 'Version v%v' (invalid per schema) -> 'Version'

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/build.gradle.kts

@@ -28,8 +28,8 @@ android {
         // which builds this version and then creates the matching vX.Y.Z tag +
         // release itself (versionCode is pinned to MAJOR*10000 + MINOR*100 +
         // PATCH from versionName, e.g. 2.7.2 -> 20702). See docs/RELEASING.md.
-        versionCode = 20703
-        versionName = "2.7.3"
+        versionCode = 20704
+        versionName = "2.7.4"
 
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
     }