[EXEC-5670] Update Xcode release & deprecation policy

[OompahLoompah]

Description

Update the Xcode release & deprecation policy with the new deprecation policy and ensure the rest of the document is consistent with current practices.

Reasons

Current practice with regards to Xcode image release and deprecation diverged from our policy, especially regarding how we announce new images.

Content Checklist

Please follow our style when contributing to CircleCI docs. Our style guide is here: https://circleci.com/docs/style/style-guide-overview.

Please take a moment to check through the following items when submitting your PR (this is just a guide so will not be relevant for all PRs):

• Break up walls of text by adding paragraph breaks.
• Consider if the content could benefit from more structure, such as lists or tables, to make it easier to consume.
• Keep the title between 20 and 70 characters.
• Consider whether the content would benefit from more subsections (h2-h6 headings) to make it easier to consume.
• Check all headings h1-h6 are in sentence case (only first letter is capitalized).
• Include relevant backlinks to other CircleCI docs/pages.