[Styles] Consolidate se23 styles in `polaris-react/src/styles`

[sophschneider]

in _common.scss we can remove $se23ShadowBevelOptOut but not $se23 since a lot of our consuming repos still use the classname and we don't want to affect their styles.

We should also add a comment above the $se23 definition telling consumers to not use and the date that it will be officially deprecated. Lets also add a calendar event or something for the day we will deprecate, and create clean up comms to consumer on the date they should finish their cleanup by

Sources:

• se23 class name used in many consuming repos internally: https://grokt.shopify.io/results?q=Polaris-Summer-Editions-2023
• shadow bevel class name only in polaris: https://grokt.shopify.io/results?q=Polaris-SE23-Shadow-Bevel-Opt-Out

Tasks:

• remove $se23ShadowBevelOptOut
• keep $se23
• determine deprecation date for $se23
• add comment above $se23 definition with warning not to use and deprecation date
• add deprecation date in our calendar

[laurkim]

Blocked by #10189.

[sophschneider]

Added an issue description for keeping the se23 classname for v12