Mention RubyGems version for source installation

What does this MR do and why?

Mention RubyGems version for source installation

See #409094 (closed)

We no longer strictly need a specific version of RubyGems, but it's still nice to mention to update it. See !119070 (comment 1373993128)

Review app:

• https://main-ee-119070.docs.gitlab-review.app/ee/install/installation.html#software-requirements
• https://main-ee-119070.docs.gitlab-review.app/ee/install/installation.html#3-rubygems

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Related to #409094 (closed)