[ci] Use the latest gcc version in macOS CI jobs

[StrikerRUS]

Noticed the following in our logs:

...

Warning: gcc 11.2.0 is already installed and up-to-date.
To reinstall 11.2.0, run:
  brew reinstall gcc

...

-- The C compiler identification is GNU 10.3.0
-- The CXX compiler identification is GNU 10.3.0

...

GitHub Actions macOS image has multiple preinstalled gcc versions.

gcc-9 (Homebrew GCC 9.4.0) 9.4.0 - available by gcc-9 alias
gcc-10 (Homebrew GCC 10.3.0) 10.3.0 - available by gcc-10 alias
gcc-11 (Homebrew GCC 11.2.0) 11.2.0 - available by gcc-11 alias
https://github.com/actions/virtual-environments/blob/main/images/macos/macos-10.15-Readme.md#language-and-runtime

This is why our master is not failing with these lines

LightGBM/.ci/test.sh

Lines 3 to 5 in a77260f

	if [[ $OS_NAME == "macos" ]] && [[ $COMPILER == "gcc" ]]; then
	export CXX=g++-10
	export CC=gcc-10

[StrikerRUS]

With this PR we compile LightGBM with gcc-11 on macOS:

...

-- The C compiler identification is GNU 11.2.0
-- The CXX compiler identification is GNU 11.2.0

...

[StrikerRUS]

Also everything is OK according to the logs with swapped compilers: bc2f8ce.

[jameslamb]

ah ok, yeah I support this! Thanks for looking into it.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.