Allow Rubocop to support Ruby 2.3.1

What does this MR do?

Update Rubocop's TargetRubyVersion to 2.3 so that new ruby features can be used without Rubocop failures.

Are there points in the code the reviewer needs to double check?

Could this change impact anyone negatively?

Why was this MR needed?

The .ruby-version is set to 2.3.1 but Rubocop will penalise the use of ruby 2.3.1 features, such as the squiggly heredoc that replaces the ActionSupport#strip_heredoc method (that I wanted to use in !7494 (merged) ).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added
• [-] Documentation created/updated
• [-] API support added
• Tests
  • [-] Added for this feature/bug
  • All builds are passing
• [-] Conform by the merge request performance guides
• [-] Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?