Cocoapodsがインストール出来ず、エラーが発生する

前提・実現したいこと

以前にインストールしたCocoapodsのアップデートを行おうと思いsudo gem update cocoapodsを実行。
上手くアップデート出来なかった為、アンインストールのコマンドsudo gem uninstall cocoapodsを実行。
アンインストール完了後、インストールのコマンドsudo gem install cocoapodsを実行するもエラーが発生。

ネットで記載されている対応策を一通り試してみましたが、同じエラーが発生します。
以前は問題なくインストール出来たのに、再インストールが出来ないのが全く分からず困っております。

どなたかご教示いただけませんでしょうか？

発生している問題・エラーメッセージ

dhcp026:~ tokyo_kaihatsu$ sudo gem install -n /usr/local/bin cocoapods
Password:
NOTE: Gem::Specification#rubyforge_project= is deprecated with no replacement. It will be removed on or after 2019-12-01.
Gem::Specification#rubyforge_project= called from /Library/Ruby/Gems/2.3.0/specifications/escape-0.0.4.gemspec:18.
NOTE: Gem::Specification#rubyforge_project= is deprecated with no replacement. It will be removed on or after 2019-12-01.
Gem::Specification#rubyforge_project= called from /Library/Ruby/Gems/2.3.0/specifications/fuzzy_match-2.0.4.gemspec:17.
NOTE: Gem::Specification#rubyforge_project= is deprecated with no replacement. It will be removed on or after 2019-12-01.
Gem::Specification#rubyforge_project= called from /Library/Ruby/Gems/2.3.0/specifications/i18n-0.9.5.gemspec:17.
NOTE: Gem::Specification#rubyforge_project= is deprecated with no replacement. It will be removed on or after 2019-12-01.
Gem::Specification#rubyforge_project= called from /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/gems/2.3.0/specifications/nokogiri-1.5.6.gemspec:22.
NOTE: Gem::Specification#rubyforge_project= is deprecated with no replacement. It will be removed on or after 2019-12-01.
Gem::Specification#rubyforge_project= called from /Library/Ruby/Gems/2.3.0/specifications/escape-0.0.4.gemspec:18.
NOTE: Gem::Specification#rubyforge_project= is deprecated with no replacement. It will be removed on or after 2019-12-01.
Gem::Specification#rubyforge_project= called from /Library/Ruby/Gems/2.3.0/specifications/i18n-0.9.5.gemspec:17.
NOTE: Gem::Specification#rubyforge_project= is deprecated with no replacement. It will be removed on or after 2019-12-01.
Gem::Specification#rubyforge_project= called from /Library/Ruby/Gems/2.3.0/specifications/fuzzy_match-2.0.4.gemspec:17.
NOTE: Gem::Specification#rubyforge_project= is deprecated with no replacement. It will be removed on or after 2019-12-01.
Gem::Specification#rubyforge_project= called from /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/gems/2.3.0/specifications/nokogiri-1.5.6.gemspec:22.
NOTE: Gem::Specification#rubyforge_project= is deprecated with no replacement. It will be removed on or after 2019-12-01.
Gem::Specification#rubyforge_project= called from /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/gems/2.3.0/specifications/nokogiri-1.5.6.gemspec:22.
Building native extensions. This could take a while...
ERROR:  Error installing cocoapods:
    ERROR: Failed to build gem native extension.

    current directory: /Library/Ruby/Gems/2.3.0/gems/ffi-1.12.2/ext/ffi_c
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby -I /Library/Ruby/Site/2.3.0 -r ./siteconf20200227-1718-x64u5b.rb extconf.rb
mkmf.rb cant find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/include/ruby.h

extconf failed, exit code 1

Gem files will remain installed in /Library/Ruby/Gems/2.3.0/gems/ffi-1.12.2 for inspection.
Results logged to /Library/Ruby/Gems/2.3.0/extensions/universal-darwin-18/2.3.0/ffi-1.12.2/gem_make.out

試したこと

・gemを最新にする（sudo gem upgrade --system）
・XCodeのCommandLineToolsをインストール

補足情報（FW/ツールのバージョンなど）

macOS　10.14.6
Xcode　11.2.1
ruby 2.3.7p456 (2018-03-28 revision 63024) [universal.x86_64-darwin18]
Gem　3.1.2