Enable `InternalImportsByDefault` in remaining modules (#2042)

This PR enables `InternalImportsByDefault` on `GRPCHealth` and
`InteroperabilityTests`.
It also regenerates the protos with the latest version of swift-protobuf
(v1.28.1), and increases its min version to 1.28.1 to make sure that we
don't get build failures when using earlier generators.

Sources/protoc-gen-grpc-swift/Options.swift

@@ -74,10 +74,16 @@ struct GeneratorOptions {
   private(set) var gRPCModuleName = "GRPC"
   private(set) var swiftProtobufModuleName = "SwiftProtobuf"
   private(set) var generateReflectionData = false
+
   #if compiler(>=6.0)
   private(set) var v2 = false
   #endif
+
+  #if compiler(>=6.0)
   private(set) var useAccessLevelOnImports = true
+  #else
+  private(set) var useAccessLevelOnImports = false
+  #endif
 
   init(parameter: any CodeGeneratorParameter) throws {
     try self.init(pairs: parameter.parsedPairs)