There are no versions of activesupport (= 7.0.3) compatible with your Ruby & RubyGems. Maybe try installing an older version of the gem you're looking for

ERROR:  Error installing rails:
	There are no versions of activesupport (= 7.0.3) compatible with your Ruby & RubyGems. Maybe try installing an older version of the gem you're looking for?
	activesupport requires Ruby version >= 2.7.0. The current ruby version is 2.6.3.62.

上記のエラーが出たので解消した手順を記す
Describe the procedure for resolving the above error

brewのアップデート
Update brew

brew update

rbenvをインストール
Install rbenv

brew install rbenv

rbenv initを実行
Run rbenv init

rbenv init

出てきた文章を読み、eval "$(rbenv init - zsh)"の部分を~/.zshrcに追記
Read the text that comes up and add the eval "$(rbenv init - zsh)" to ~/.zshrc

# Load rbenv automatically by appending
# the following to ~/.zshrc:

eval "$(rbenv init - zsh)"

~/.zshrcの再読み込み
Reload ~/.zshrc

source ~/.zshrc

rbenvで2.7系のrubyをインストール
Install ruby 2.7 series with rbenv

rbenv install 2.7.6

rbenv2.7.6を実際に使うように設定
Configure rbenv2.7.6 to actually use

rbenv local 2.7.6

rubyのバージョン確認
Check ruby version

ruby -v

これで2.7.6になってたらおｋ！
If it's 2.7.6, you're good to go!