Update deprecation terms

What does this MR do and why?

Following on from the conversation in #352769 (closed), this MR serves as a proposal to add clarity to our deprecation process and definitions.

@gl-product-leadership

Follow-up requirements

• Update text on GitLab the Product @brhea (Draft MR)
• Update deprecation and removal MR templates and content samples !89182 (merged) @brhea
• Create proposal for flagging a feature in "End of Support" gitlab-com/www-gitlab-com!105785 (merged) @brhea

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.