chore: Update UPDATING.md

[john-bodley]

SUMMARY

This PR cleans up UPDATING.md in preparation for the Superset 2.0 release. This includes:

1. Ordering the PRs in reverse chronological order.
2. Standardizing (as best as possible) the verbiage.
3. Adding links (where appropriate).
4. Other cleanup.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #19480 (2de18c0) into master (4513cc4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #19480   +/-   ##
=======================================
  Coverage   66.54%   66.54%           
=======================================
  Files        1692     1692           
  Lines       64791    64791           
  Branches     6661     6661           
=======================================
  Hits        43117    43117           
  Misses      19974    19974           
  Partials     1700     1700           

Flag	Coverage Δ
hive	52.86% <ø> (ø)
javascript	51.24% <ø> (ø)
mysql	81.90% <ø> (ø)
postgres	81.95% <ø> (ø)
presto	52.71% <ø> (ø)
python	82.37% <ø> (ø)
sqlite	81.71% <ø> (ø)
unit	47.94% <ø> (ø)

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[srinify]

Approved! Some merge conflicts for you to quickly resolve @john-bodley 🙏