Add missing availability annotations (grpc#1696)

Author: glbrntt

Sources/GRPCCore/Internal/Result+Catching.swift

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 extension Result where Failure == any Error {
   /// Like `Result(catching:)`, but `async`.
   ///

Tests/GRPCCoreTests/Internal/Result+CatchingTests.swift

@@ -18,6 +18,7 @@ import XCTest
 
 @testable import GRPCCore
 
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 final class ResultCatchingTests: XCTestCase {
   func testResultCatching() async {
     let result = await Result {