docs: Add potential downtime line item to UPDATING.md

[john-bodley]

SUMMARY

Add potential downline line item to UPDATING.md in relation to #21284 which contains a potentially locking DDL operation.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

CI.

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 #22130 (f046d23) into master (06f87e1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #22130   +/-   ##
=======================================
  Coverage   66.97%   66.97%           
=======================================
  Files        1832     1832           
  Lines       69905    69905           
  Branches     7570     7570           
=======================================
  Hits        46820    46820           
  Misses      21127    21127           
  Partials     1958     1958           

Flag	Coverage Δ
hive	52.57% <ø> (ø)
javascript	53.77% <ø> (ø)
mysql	78.13% <ø> (ø)
postgres	78.19% <ø> (ø)
presto	52.46% <ø> (ø)
python	81.36% <ø> (ø)
sqlite	76.65% <ø> (ø)
unit	50.82% <ø> (ø)

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