deps [nfc]: Update description of Flutter version plans

We don't plan to switch to "beta" or "stable" versions -- better to
live at head, by sticking to the main channel.

(We still want to pin the precise version, but let the issue track
that.  The pubspec here isn't what would control a pin anyway.)

Also point to our instructions for upgrading these versions.

Author: gnprice

pubspec.yaml

@@ -11,11 +11,9 @@ publish_to: 'none'
 version: 0.0.19+19
 
 environment:
-  # We use a development version of Flutter while in beta,
-  # and the corresponding dev version of the Dart SDK.  The hope is
-  # that by the time we want to release, these will have become stable.
-  # TODO: Before general release, switch to stable Flutter and Dart versions,
-  #   or pin exact versions: https://github.com/zulip/zulip-flutter/issues/15
+  # We use a recent version of Flutter from its main channel, and
+  # the corresponding recent version of the Dart SDK.
+  # Feel free to update these regularly; see README.md for instructions.
   sdk: '>=3.6.0-149.3.beta <4.0.0'
   flutter: '>=3.25.0-1.0.pre.41'