Release v2.20.3

Bump versionName to 2.20.3 (versionCode 22003, per the MAJOR*10000 +
MINOR*100 + PATCH rule), promote the CHANGELOG's [Unreleased] section to
[2.20.3], and hand-write the per-version "What's New" for all twelve
store locales.

The release carries one user-visible fix: the French agenda-range plural
crash reported as #297 and #298.

2.20.0, 2.20.1 and 2.20.2 each shipped with only an en-US changelog, so
every other locale published nothing; this restores the full coverage
2.19.1 had (ar, de-DE, en-GB, en-US, es-ES, fr-FR, it-IT, pl-PL, pt-BR,
pt-PT, ru-RU, zh-CN). en-GB matters on its own because it, not en-US, is
the Play Console's default locale and what Play falls back to. Every file
is well inside Play's 500-character cap — the longest is it-IT at 348 —
and scripts/check_changelog_lengths.sh passes.

Per docs/RELEASING.md this bump is what triggers release.yaml once the
branch reaches main, so the on-device gate (step 4, scripts/verify-release.sh)
still has to pass before that merge.

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 = 22002
-        versionName = "2.20.2"
+        versionCode = 22003
+        versionName = "2.20.3"
 
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
     }