Enable Swift 6 language mode for a number of v2 modules (#1959)

Enable Swift 6 language mode for a number of v2 modules

Motivation:

v2 will eventually require the v6 language, doing this in one go is a
reasonably large change so adopt a few targets at a time.

Modifications:

- Fixup warnings/errors in the following targets:
  - GRPCInterceptors
  - GRPCInProcessTransport
  - GRPCHTTP2Core
  - GRPCHTTP2TransportNIOPosix
  - GRPCHTTP2TransportNIOTransportServices
  - GRPCCodeGen
  - GRPCProtobufCodeGenTests
  - GRPCProtobufCodeGen
  - GRPCProtobufTests
  - GRPCProtobuf
  - interoperability-tests
  - performance-worker
- Fixup warnings/errors but remain in the v5 language mode for the
  following:
  - GRPCInterceptorsTests

Result:

More modules use v6 language mode

Tests/GRPCProtobufTests/ProtobufCodingTests.swift

@@ -76,7 +76,7 @@ final class ProtobufCodingTests: XCTestCase {
 struct TestMessage: SwiftProtobuf.Message {
   var text: String = ""
   var unknownFields = SwiftProtobuf.UnknownStorage()
-  static var protoMessageName: String = "Test" + ".ServiceRequest"
+  static let protoMessageName: String = "Test.ServiceRequest"
   init() {}
 
   mutating func decodeMessage<D>(decoder: inout D) throws where D: SwiftProtobuf.Decoder {