homebrewのupdateでエラー

MacOS El Capitan 10.11.6
Homebrew 2.5.6

nodebrewをインストールしたく、homebrewをupdateすると、エラーが出ました。
そこでhomebrew doctorしたところWarningがたくさん出て何から試したら良いのか分かりません。

上から２番目のWarningは、OSのバージョンが古い、３番目は、upgradeしなさい、と言ってるのでしょうか？
初めてでよく分かりません。

先輩方教えてください。宜しくお願いします。

$ brew doctor
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!

Warning: You have the following deprecated, official taps tapped:
  Homebrew/homebrew-versions
Untap them with `brew untap`.

Warning: You are using macOS 10.11.
We (and Apple) do not provide support for this old version.
You will encounter build failures with some formulae.
Please create pull requests instead of asking for help on Homebrew's GitHub,
Discourse, Twitter or IRC. You are responsible for resolving any issues you
experience while you are running this old version.


Warning: An outdated version (2.10.1) of Git was detected in your PATH.
Git 2.14.3 or newer is required for Homebrew.
Please upgrade:
  brew install git

Warning: Broken symlinks were found. Remove them with `brew cleanup`:
  /usr/local/share/man/man1/brew-cask.1