[libomptarget] Drop remote plugin cmake version requirement to match llvm

[JonChesterfield]

Summary

LLVM docs at https://llvm.org/docs/CMake.html#quick-start state 3.13.4

openmp/libomptarget/plugins/remote/src/CMakeLists.txt

 ##===----------------------------------------------------------------------===##
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 ##===----------------------------------------------------------------------===##
 #
 # Build a plugin for remote offloading.
 #
 ##===----------------------------------------------------------------------===##
 
-cmake_minimum_required(VERSION 3.15)
+cmake_minimum_required(VERSION 3.13.4)
 
 # Define the suffix for the runtime messaging dumps.
 add_definitions(-DTARGET_NAME=RPC)
 
 include_directories(${LIBOMPTARGET_SRC_DIR})
 include_directories(${LIBOMPTARGET_INCLUDE_DIR})
 include_directories(${GRPC_INCLUDE_DIR})
 include_directories(${RPC_INCLUDE_DIR})