Tracer.withSpan's isolation param should be 'isolated'

**Motivation:**

The intent of this parameter is to not cross an isolation boundary when
calling withSpan, thus, we were missing the isolated modifier in this
protocol.

**Modifications:**

Make the `isolation: (any Actor)? = #isolation,` parameter 'isolated',
as was originally intended

**Result:**

Not crossing actor boundary when calling this method means less errors
when adopting Swift 6 mode, and correct behavior of these functions

Author: ktoso

Sources/Tracing/TracerProtocol+Legacy.swift

@@ -404,7 +404,7 @@ extension LegacyTracer {
         _ operationName: String,
         context: @autoclosure () -> ServiceContext = .current ?? .topLevel,
         ofKind kind: SpanKind = .internal,
-        isolation: (any Actor)? = #isolation,
+        isolation: isolated (any Actor)? = #isolation,
         function: String = #function,
         file fileID: String = #fileID,
         line: UInt = #line,
@@ -610,7 +610,7 @@ extension Tracer {
         at instant: @autoclosure () -> some TracerInstant = DefaultTracerClock.now,
         context: @autoclosure () -> ServiceContext = .current ?? .topLevel,
         ofKind kind: SpanKind = .internal,
-        isolation: (any Actor)? = #isolation,
+        isolation: isolated (any Actor)? = #isolation,
         function: String = #function,
         file fileID: String = #fileID,
         line: UInt = #line,

Sources/Tracing/TracerProtocol.swift

@@ -263,7 +263,7 @@ extension Tracer {
         _ operationName: String,
         context: @autoclosure () -> ServiceContext = .current ?? .topLevel,
         ofKind kind: SpanKind = .internal,
-        isolation: (any Actor)? = #isolation,
+        isolation: isolated (any Actor)? = #isolation,
         function: String = #function,
         file fileID: String = #fileID,
         line: UInt = #line,
@@ -353,7 +353,7 @@ extension Tracer {
         context: @autoclosure () -> ServiceContext = .current ?? .topLevel,
         ofKind kind: SpanKind = .internal,
         at instant: @autoclosure () -> some TracerInstant = DefaultTracerClock.now,
-        isolation: (any Actor)? = #isolation,
+        isolation: isolated (any Actor)? = #isolation,
         function: String = #function,
         file fileID: String = #fileID,
         line: UInt = #line,