macOS pod install fails

Nobody has claimed this yet.

```
Resolving dependencies of `Podfile`
CDN: trunk Relative path: CocoaPods-version.yml exists! Returning local because checking is only perfomed in repo update
CDN: trunk Relative path: all_pods_versions_0_3_5.txt exists! Returning local because checking is only perfomed in repo update
CDN: trunk Relative path: Specs/0/3/5/Firebase/7.3.0/Firebase.podspec.json exists! Returning local because checking is only perfomed in repo update
CDN: trunk Relative path: Specs/0/3/5/Firebase/6.33.0/Firebase.podspec.json exists! Returning local because checking is only perfomed in repo update
CDN: trunk Relative path: Specs/0/3/5/Firebase/6.33.0/Firebase.podspec.json exists! Returning local because checking is only perfomed in repo update
CDN: trunk Relative path: all_pods_versions_c_7_9.txt exists! Returning local because checking is only perfomed in repo update
CDN: trunk Relative path: Specs/c/7/9/Reachability/3.2/Reachability.podspec.json exists! Returning local because checking is only perfomed in repo update
CDN: trunk Relative path: Specs/0/3/5/Firebase/6.33.0/Firebase.podspec.json exists! Returning local because checking is only perfomed in repo update
CDN: trunk Relative path: Specs/0/3/5/Firebase/6.33.0/Firebase.podspec.json exists! Returning local because checking is only perfomed in repo update
CDN: trunk Relative path: Specs/0/3/5/Firebase/6.33.0/Firebase.podspec.json exists! Returning local because checking is only perfomed in repo update
[!] CocoaPods could not find compatible versions for pod "flutter_web_auth":
In Podfile:
flutter_web_auth (from `Flutter/ephemeral/.symlinks/plugins/flutter_web_auth/macos`)

Specs satisfying the `flutter_web_auth (from `Flutter/ephemeral/.symlinks/plugins/flutter_web_auth/macos`)` dependency were found, but they required a higher minimum deployment target.
```

I am not sure the Firebase dependencies have anything to do with it.

What I noticed is that `flutter_web_auth` declares the following in the Podspec:

```
s.platform = :osx, '10.15'
```

While the other plugins use `s.platform = :osx, '10.11'` instead.

Start with the flutter_web_auth macOS Podspec and compare its 10.15 deployment target with the 10.11 targets used by the other plugins. Reproduce the macOS pod install failure, then verify that dependency resolution succeeds with a compatible deployment target.

Written by the indexing model from the issue text.