I have faced the same issue for Ruby 2.2.2 and many gems were dependent on that. So I have created a docker container for ubuntu 18.04 and then installed ruby on it. It's working.

In order to make installing of Ruby versions 2.6.x or 2.7.x successful on M1 MacBook using either rbenv or asdf (asdf is used in this example) follow these steps:

Upgrade to the latest version of rbenv or asdf-ruby plugin using your prefered installation method. In my case it's asdf-ruby installed over homebrew:

brew upgrade asdf
asdf plugin update ruby

Reinstall the current versions of openssl, readline and ruby-build in order to have the latest versions and configs:

brew uninstall --ignore-dependencies readline
brew uninstall --ignore-dependencies openssl
brew uninstall --ignore-dependencies ruby-build
rm -rf /opt/homebrew/etc/openssl@1.1
brew install -s readline
brew install -s openssl
brew install -s ruby-build

In your shell config .bashrc or .zshrc add the following ENV variables:

export RUBY_CONFIGURE_OPTS="--with-openssl-dir=$(brew --prefix openssl@1.1)"
export LDFLAGS="-L/opt/homebrew/opt/readline/lib:$LDFLAGS"
export CPPFLAGS="-I/opt/homebrew/opt/readline/include:$CPPFLAGS"
export PKG_CONFIG_PATH="/opt/homebrew/opt/readline/lib/pkgconfig:$PKG_CONFIG_PATH"
export optflags="-Wno-error=implicit-function-declaration"
export LDFLAGS="-L/opt/homebrew/opt/libffi/lib:$LDFLAGS"
export CPPFLAGS="-I/opt/homebrew/opt/libffi/include:$CPPFLAGS"
export PKG_CONFIG_PATH="/opt/homebrew/opt/libffi/lib/pkgconfig:$PKG_CONFIG_PATH"

This will ensure that the proper libraries and headers are used during the installations and it will ignore the implicit-function-declaration that is preventing some versions to continue installation. Note that for some other shells like fish the exporting of these variables will be a bit different.

Now start a new terminal session and you can try installing the older ruby versions:

asdf install ruby 2.7.2
asdf install ruby 2.6.5

Note that really old versions below 2.5 might still have issues. Most of the credits go to this Github issue.

UPDATE

For Ruby 2.2 please change the following variable:

export RUBY_CONFIGURE_OPTS=openssl@1.0

And do a

asdf reshim ruby

Thanks @xjlin0 for this update