build: bump github.com/firebase/firebase-ios-sdk from 12.5.0 to 12.6.0 (#107)

Bumps [github.com/firebase/firebase-ios-sdk](https://github.com/firebase/firebase-ios-sdk) from 12.5.0 to 12.6.0.
- [Release notes](https://github.com/firebase/firebase-ios-sdk/releases)
- [Commits](firebase/firebase-ios-sdk@12.5.0...12.6.0)

---
updated-dependencies:
- dependency-name: github.com/firebase/firebase-ios-sdk
  dependency-version: 12.6.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]

Package.resolved

@@ -16,7 +16,7 @@ let package = Package(
     dependencies: [
         .package(
             url: "https://github.com/firebase/firebase-ios-sdk.git",
-            exact: "12.5.0"
+            exact: "12.6.0"
         ),
         .package(
             url: "https://github.com/govuk-one-login/mobile-ios-networking.git",