Update X Ads API analytics documentation to correct outdated information on analytics.mdx

  - Update API version references from /9/ and /11/ to /12/ across all example requests (async analytics, active entities guide)
  - Update example dates from 2017/2019 to 2025 across all request/response samples (sync analytics, async analytics, reach, active entities)
  - Remove discontinued Vine references from media_views and media_engagements metric descriptions
  - Fix FAQ placements guidance: "both placements" → "all placements" to reflect all four values (ALL_ON_TWITTER, PUBLISHER_NETWORK, SPOTLIGHT, TREND)
  - Update retweets metric description to "reposts" per X rebrand
  - Update prose references from "promoted Tweets" to "promoted posts" (API enum values like PROMOTED_TWEET left unchanged)
  - Simplify async analytics download example filename
  - Add <!-- UPDATED --> comment markers at each change for reviewer visibility

Signed-off-by: dianalauragarcia <dianalaurag@x.ai>

Author: dianalauragarcia