Align supported Swift versions with rest of the ecosystem (#141)

# Motivation

Most of the server ecosystem follows the supported Swift versions from `swift-nio`. This means supporting the latest 3 Swift releases.

# Modification

This PR drops support for all Swift versions before 5.8.

# Result

This makes maintaining this repository easier and allows us to clean up some no longer needed stuff.

Author: FranzBusch

[email protected]

@@ -1253,7 +1253,6 @@ public final class NOOPMetricsHandler: MetricsFactory, CounterHandler, FloatingP
 
 // MARK: - Sendable support helpers
 
-#if compiler(>=5.6)
 extension MetricsSystem: Sendable {}
 extension Counter: Sendable {}
 extension FloatingPointCounter: Sendable {}
@@ -1263,10 +1262,5 @@ extension Timer: Sendable {}
 extension Meter: Sendable {}
 // ideally we would not be using @unchecked here, but concurrency-safety checks do not recognize locks
 extension AccumulatingRoundingFloatingPointCounter: @unchecked Sendable {}
-#endif
 
-#if compiler(>=5.6)
 @preconcurrency public protocol _SwiftMetricsSendableProtocol: Sendable {}
-#else
-public protocol _SwiftMetricsSendableProtocol {}
-#endif

[email protected]

@@ -75,7 +75,6 @@ extension Timer {
     }
 }
 
-#if swift(>=5.7)
 extension Timer {
     /// Convenience for recording a duration based on ``Duration``.
     ///
@@ -99,4 +98,3 @@ extension Timer {
         self.recordNanoseconds(nanoseconds.partialValue)
     }
 }
-#endif

[email protected]

@@ -576,26 +576,16 @@ extension NSLock {
 
 // MARK: - Errors
 
-#if compiler(>=5.6)
 public enum TestMetricsError: Error {
     case missingMetric(label: String, dimensions: [(String, String)])
     case illegalMetricType(metric: Sendable, expected: String)
 }
 
-#else
-public enum TestMetricsError: Error {
-    case missingMetric(label: String, dimensions: [(String, String)])
-    case illegalMetricType(metric: Any, expected: String)
-}
-#endif
-
 // MARK: - Sendable support
 
-#if compiler(>=5.6)
 // ideally we would not be using @unchecked here, but concurrency-safety checks do not recognize locks
 extension TestMetrics: @unchecked Sendable {}
 extension TestCounter: @unchecked Sendable {}
 extension TestMeter: @unchecked Sendable {}
 extension TestRecorder: @unchecked Sendable {}
 extension TestTimer: @unchecked Sendable {}
-#endif