Add back support for testing Swift 5.7

Author: tyiu

.github/workflows/unit.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: macos-13
     strategy:
       matrix:
-        swift: ['5.8', '5.9']
+        swift: ['5.7', '5.8', '5.9']
 
     steps:
     - name: Checkout

README.md

@@ -6,7 +6,7 @@ Nostr SDK for Apple Platforms is a native Swift library that enables developers
 
 ## Minimum Requirements
 
-- Swift 5.8
+- Swift 5.7
 - iOS 15
 - macOS 13