[ci] Use the latest gcc version in macOS CI jobs (#4640)

* Use the latest gcc version in macOS CI jobs

* test: swap compilers

* Revert "test: swap compilers"

* Revert "test: swap compilers"

.ci/test.sh

 #!/bin/bash
 
 if [[ $OS_NAME == "macos" ]] && [[ $COMPILER == "gcc" ]]; then
-    export CXX=g++-10
-    export CC=gcc-10
+    export CXX=g++-11
+    export CC=gcc-11
 elif [[ $OS_NAME == "linux" ]] && [[ $COMPILER == "clang" ]]; then
     export CXX=clang++
     export CC=clang