Update docs to require Ruby 2.7

Ruby 2.6 does not work anymore due to keyword argument changes.

Bundler v2 comes standard with Ruby 2.7, so we no longer need to install
it manually.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/287799

doc/install/installation.md

@@ -224,12 +224,6 @@ make
 sudo make install
 ```
 
-Then install the Bundler gem (a version below 2.x):
-
-```shell
-sudo gem install bundler --no-document --version '< 2'
-```
-
 ## 3. Go
 
 In GitLab 8.0 and later, GitLab has several daemons written in Go. To install

doc/update/upgrading_from_source.md

@@ -61,8 +61,8 @@ sudo service gitlab stop
 
 ### 3. Update Ruby
 
-NOTE: Beginning in GitLab 12.2, we only support Ruby 2.6 or higher, and dropped
-support for Ruby 2.5. Be sure to upgrade if necessary.
+NOTE: Beginning in GitLab 13.6, we only support Ruby 2.7 or higher, and dropped
+support for Ruby 2.6. Be sure to upgrade if necessary.
 
 You can check which version you are running with `ruby -v`.
 
@@ -70,21 +70,15 @@ Download Ruby and compile it:
 
 ```shell
 mkdir /tmp/ruby && cd /tmp/ruby
-curl --remote-name --progress https://cache.ruby-lang.org/pub/ruby/2.6/ruby-2.6.6.tar.gz
-echo '2d78048e293817f38d4ede4ebc7873013e97bb0b  ruby-2.6.6.tar.gz' | shasum -c - && tar xzf ruby-2.6.6.tar.gz
-cd ruby-2.6.6
+curl --remote-name --progress https://cache.ruby-lang.org/pub/ruby/2.7/ruby-2.7.2.tar.gz
+echo 'cb9731a17487e0ad84037490a6baf8bfa31a09e8  ruby-2.7.2.tar.gz' | shasum -c - && tar xzf ruby-2.7.2.tar.gz
+cd ruby-2.7.2
 
 ./configure --disable-install-rdoc
 make
 sudo make install
 ```
 
-Install Bundler:
-
-```shell
-sudo gem install bundler --no-document --version '< 2'
-```
-
 ### 4. Update Node.js
 
 NOTE: To check the minimum required Node.js version, see [Node.js versions](../install/requirements.md#nodejs-versions).