Show outdated CocoaPods version in hint text

[jmagman]

Description

Was:

CocoaPods out of date (1.6.0 is recommended).

Now:

CocoaPods 1.4.0 out of date (1.6.0 is recommended).

Related Issues

Fixes #40116.

Tests

Updated CocoaPods validator test.

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change (Please read [Handling breaking changes]). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.

[christopherfujino]

nice catch, lgtm

[codecov]

Codecov Report

Merging #40117 into master will decrease coverage by 0.89%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #40117     +/-   ##
=========================================
- Coverage   58.94%   58.05%   -0.9%     
=========================================
  Files         192      194      +2     
  Lines       18442    18727    +285     
=========================================
+ Hits        10871    10872      +1     
- Misses       7571     7855    +284

Flag	Coverage Δ
#flutter_tool	58.05% <100%> (-0.9%)	⬇️

Impacted Files	Coverage Δ
...utter_tools/lib/src/macos/cocoapods_validator.dart	82.6% <100%> (+0.79%)	⬆️
...ages/flutter_tools/lib/src/base/user_messages.dart	49.05% <100%> (+7.54%)	⬆️
packages/flutter_tools/lib/src/base/utils.dart	81.18% <0%> (-13.37%)	⬇️
...lib/src/build_runner/web_compilation_delegate.dart	14.15% <0%> (-11.24%)	⬇️
packages/flutter_tools/lib/src/web/compile.dart	78.78% <0%> (-5.59%)	⬇️
packages/flutter_tools/lib/src/compile.dart	71.57% <0%> (-5.36%)	⬇️
packages/flutter_tools/lib/src/devfs.dart	65.51% <0%> (-2.96%)	⬇️
packages/flutter_tools/lib/src/version.dart	90.9% <0%> (-1.92%)	⬇️
...ckages/flutter_tools/lib/src/base/file_system.dart	67.85% <0%> (-1.79%)	⬇️
packages/flutter_tools/lib/src/cache.dart	43.5% <0%> (-0.73%)	⬇️
... and 17 more

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