Check frozen string in style builds

Enable frozen string literals in:

Enables RuboCop::Cop::Style::FrozenStringLiteralComment in static-analysis builds to begin enforcing efforts done for #47424 (closed).

The current linting affects only the app/ directory. All other files listed in the exclusions were added after running rubocop locally.

Why was this MR needed?

Because frozen strings require fewer memory allocations, and memory efficiency is a good thing.

What are the relevant issue numbers?

Partially addresses #47424 (closed).

Suggested Reviewers/Approvers/Mergers

@stanhu @rymai @godfat