Enable RSpec/MultipleMemoizedHelpers by default

What does this MR do?

Enable RSpec/MultipleMemoizedHelpers by default

We bring this cop into the main config, and set a lower threshold.

Some files are excluded (migration specs, which we don't want to touch), others are fixed, and some have explicit disablements.

The threshold of 28 is chosen as it accounts for the vast majority of our usage. We should aim to bring this down to 20, and eventually closer to 10.

Note that at present let_it_be is not counted.

Does this MR meet the acceptance criteria?

Conformity

As a developer facing change, no changelog entry is needed.

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.