Refactor specs around ssh_keys in users_controller_spec.rb

What does this MR do?

Refactors specs around ssh_keys in users_controller_spec.rb, which have existed for years but were recently pointed out in review process of !48991 (merged) by @dbalexandre.

Also fixes a potential bug in the gpg_keys spec discussed at !48321 (diffs, comment 470562594)

This MR contains:

• Use single quotation marks instead of double
• Use context instead of describe
• Use shared_examples_for instead of the duplicate tests

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Style guides
• Separation of EE specific content

Closes #292680 (closed)