release: 1.2.0 — new look, group admins, calls

Bumps the app to 1.2.0 (build 23) and adds the changelog entry.

The iOS export compliance flag is now false: Šepot uses only standard,
publicly available encryption (the Signal protocol via libsignal and
SQLCipher), which qualifies for the mass-market exemption and needs no
export compliance documentation from Apple.

app/ios/Runner/Info.plist

@@ -43,7 +43,7 @@
 		<key>CFBundleVersion</key>
 		<string>$(FLUTTER_BUILD_NUMBER)</string>
 		<key>ITSAppUsesNonExemptEncryption</key>
-		<true/>
+		<false/>
 		<key>LSRequiresIPhoneOS</key>
 		<true/>
 		<key>UILaunchStoryboardName</key>