You DO NOT need to upgrade Xcode.
Just open the file /usr/local/Homebrew/Library/Homebrew/extend/os/mac/diagnostic.rb ,
then remove this line check_xcode_minimum_version in the following function.
def fatal_build_from_source_checks
%w[
check_xcode_license_approved
check_xcode_minimum_version //<-- this one
check_clt_minimum_version
check_if_xcode_needs_clt_installed
].freeze
end
Then brew install should works fine.