Update rails Banner component

What does this MR do and why?

Change the default Banner background to gray and h1 tag to h2 to match the original component.

Screenshots or screen recordings

before	after	Pajamas

How to set up and validate locally

You can check the rails component on Lookbook /rails/lookbook/inspect/pajamas/banner/default and compare it with Pajamas component.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

#430244 (closed)