Update bundler to 2.3.6 because 2.1.4 is too old

What does this MR do and why?

Update bundler to 2.3.6 because 2.1.4 is too old

It was updated to 2.3.5 in !78298 (merged) but then reverted due to errors and now we try to upgrade to 2.3.6 which fixed the errors we're seeing.

• GitLab Shell: gitlab-shell!554 (merged)

See #350541 (closed)

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