Avoid installing Bundler 2.0 or higher on source-based install

What does this MR do?

Prevents GitLab administrators to install Bundler 2.0 or higher when installing via source.

Related issues

Closes #55979 (closed)

Author's checklist

• Apply the correct labels and milestone
• [n/a] Crosslink the document from the higher-level index
• [n/a] Crosslink the document from other subject-related docs
• [n/a] Feature moving tiers? Make sure the change is also reflected in features.yml
• [n/a] Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards