Update Swift tools version from 5.3 to 5.5

Notice that this introduces an error when running `swift test`:

```
$ swift test

Building for debugging...
/Users/gio/Developer/a8c/tracks-apple-pod/Sources/Model/ObjC/Common/Core Data/TracksContextManager.m:24:24: error: use of undeclared identifier 'SWIFTPM_MODULE_BUNDLE'
    NSBundle *bundle = SWIFTPM_MODULE_BUNDLE;
                       ^
1 error generated.
[0/6] Compiling TracksContextManager.m
error: fatalError
```

This is a known Swift Package Manager issue. See:

- swiftlang/swift-package-manager#4500
- https://forums.swift.org/t/5-3-resources-support-not-working-on-with-swift-test/40381

However, `bundle exec fastlane test`, which uses `xcodebuild` under the
hood, works.

Given our workflow is based on Xcode locally and Fastlane in CI, the
regression seems acceptable.

The reason I'm keen to upgrade to 5.5 is that it's the required tooling
version to integrate Buildkite's Test Analytics. See
https://github.com/buildkite/test-collector-swift/blob/v0.1.0/Package.swift#L1

Author: mokagio

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.3
+// swift-tools-version:5.5
 // The swift-tools-version declares the minimum version of Swift required to build this package.
 
 import PackageDescription