Release 2.18.1

Bump versionName/versionCode, date the CHANGELOG section, and hand-write the
What's New for every shipped language — including en-GB, which is Play's
default locale and therefore where its fallback looks.

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 = 21800
-        versionName = "2.18.0"
+        versionCode = 21801
+        versionName = "2.18.1"
 
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
     }