deps: Hold back firebase_core at <3.4.0

The upgrade to the Dart package firebase_core 3.4.0 (from 3.3.0)
leads to the CocoaPods pod Firebase/CoreOnly 11.0.0 (from 10.29.0)
and the further pod Firebase/Messaging 11.0.0 (from 10.29.0).
That produces the following `pod update` error message when
attempting to run `tools/upgrade pub`:

```
+ pod update --project-directory=macos/
Update all pods
Updating local specs repositories
Analyzing dependencies
firebase_core: Using Firebase SDK version '11.0.0' defined in 'firebase_core'
firebase_messaging: Using Firebase SDK version '11.0.0' defined in 'firebase_core'
[!] CocoaPods could not find compatible versions for pod "Firebase/Messaging":
  In Podfile:
    firebase_messaging (from `Flutter/ephemeral/.symlinks/plugins/firebase_messaging/macos`) was resolved to 15.1.0, which depends on
      Firebase/Messaging (~> 11.0.0)

Specs satisfying the `Firebase/Messaging (~> 11.0.0)` dependency were found, but they required a higher minimum deployment target.
```

Fundamentally that should be fixable, but with some effort just now
I wasn't able to actually increase that minimum deployment target
in a way that satisfied CocoaPods so as to resolve this error.
So, filed zulip#1116 for that.  Meanwhile, let our other upgrades proceed.

pubspec.yaml

@@ -42,7 +42,7 @@ dependencies:
   device_info_plus: ^10.0.1
   drift: ^2.5.0
   file_picker: ^8.0.0+1
-  firebase_core: ^3.1.0
+  firebase_core: 3.3.0  # TODO(#1116) upgrade
   firebase_messaging: ^15.0.1
   flutter_color_models: ^1.3.3+2
   html: ^0.15.1