Roll buildroot to pick up iOS 12 (#49542)

https://github.com/flutter/buildroot/pull/804
https://github.com/flutter/buildroot/pull/806
https://github.com/flutter/buildroot/pull/808
https://github.com/flutter/buildroot/pull/809
https://github.com/flutter/buildroot/pull/810

Engine tests were already running on iOS 13.

Part of https://github.com/flutter/flutter/issues/140474

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
diff --git a/DEPS b/DEPS
index 85b0b66..1497798 100644
--- a/DEPS
+++ b/DEPS
@@ -267,7 +267,7 @@
 ]
 
 deps = {
-  'src': 'https://github.com/flutter/buildroot.git' + '@' + '0141e94b34cbd3db77f28796587334857fa985da',
+  'src': 'https://github.com/flutter/buildroot.git' + '@' + '8c274b21f1ad4f2aec0a5e0ae8f4264393045b4b',
 
   'src/flutter/third_party/rapidjson':
    Var('flutter_git') + '/third_party/rapidjson' + '@' + 'ef3564c5c8824989393b87df25355baf35ff544b',