Pre-Request Checklist

• Passes rubocop code analysis (try rubocop --auto-correct)
• Tests added or adapted (try rake test)
• Changes are reflected in the documentation
• User-visible changes appended to CHANGELOG.md

Description

• Closes: Coding style for regexps in oxidized #3261
• Activates Rubocop Lint/AmbiguousRegexpLiteral, with exception of the models.
• The models keep their DSL-Syntax and we accept the ambiguity between regexp and two divisions there - ruby chooses the regexp. For this, we disable warnings in minitest (rake test). Corrects modesls without DSL-Syntax for the prompt.
• Most of the regexp in oxidized follow the style slashes with inner slashes allowed. Activate rubocop Style/RegexpLiteral with these options and correct the few regexp not following this style.

No documentation in CHANGELOG or the documentation as there is no modification for users or authors, just two new cops were activated, the comments in the code should be enough.