Update issue templates (#17250)

The "about" text was apparently too long.

.github/ISSUE_TEMPLATE/ACTIVATION.md

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 name: I am having difficulty installing Flutter or getting it to work
-about: You have run into problems while downloading or installing Flutter,
-  or the "flutter" tool is crashing, or you are running into some other
-  issue before even being able to use "flutter run".
+about: You have run into problems while downloading or installing Flutter, or the
+  "flutter" tool is crashing, or you are running into some other issue before even
+  being able to use "flutter run".
 
 ---
 

.github/ISSUE_TEMPLATE/BUG.md

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 name: I have a problem with my Flutter application.
-about: You are writing an application with Flutter but a widget is rendering
-  incorrectly, the application is crashing, there's an exception in the logs,
-  a widget is behaving badly, or some other such problem is manifesting.
+about: You are writing an application with Flutter but the application is crashing
+  or throws an exception, a widget is buggy, or something looks wrong.
 
 ---