Restore compatibility back to Swift 5.5 (#2)

There was no reason for the minimum Swift version to be bumped so high,
let's put it back down to 5.5. This should also allow the Vapor toolbox
to retain compatibility with Xcode 13.2.

.github/workflows/test.yml

@@ -9,3 +9,22 @@ on:
 jobs:
   unit-tests:
      uses: vapor/ci/.github/workflows/run-unit-tests.yml@main
+
+  unit-tests-old:
+    if: ${{ !(github.event.pull_request.draft || false) }}
+    strategy:
+      fail-fast: false
+      matrix:
+        swift-image:
+          - swift:5.5-focal
+          - swift:5.6-focal
+          - swift:5.7-jammy
+    runs-on: ubuntu-latest
+    container: ${{ matrix.swift-image }}
+    timeout-minutes: 60
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v4
+      - name: Run unit tests
+        run: SWIFT_DETERMINISTIC_HASHING=1 swift test --sanitize=thread
+

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.7
+// swift-tools-version:5.5
 import PackageDescription
 
 let package = Package(