Update the Dart language version in the pubspec generated by the dartdoc script (#116789)

dev/bots/docs.sh

@@ -59,7 +59,7 @@ function create_docset() {
   dashing_pid=$!
   wait $dashing_pid && \
   cp ./doc/flutter/static-assets/favicon.png ./flutter.docset/icon.png && \
-  "$DART" --no-sound-null-safety --disable-dart-dev --enable-asserts ./dashing_postprocess.dart && \
+  "$DART" --disable-dart-dev --enable-asserts ./dashing_postprocess.dart && \
   tar cf flutter.docset.tar.gz --use-compress-program="gzip --best" flutter.docset
   if [[ $? -ne 0 ]]; then
       >&2 echo "Dashing docset generation failed"

dev/tools/dartdoc.dart

@@ -59,7 +59,7 @@ Future<void> main(List<String> arguments) async {
   buf.writeln('homepage: https://flutter.dev');
   buf.writeln('version: 0.0.0');
   buf.writeln('environment:');
-  buf.writeln("  sdk: '>=2.10.0 <4.0.0'");
+  buf.writeln("  sdk: '>=2.12.0 <4.0.0'");
   buf.writeln('dependencies:');
   for (final String package in findPackageNames()) {
     buf.writeln('  $package:');