refactor: adopt --app as celery global option

[john-bodley]

SUMMARY

This PR updates the documentation for Celery flower which was not captured in #15053 for ensuring that the documentation is both Celery 4.x and 5.x compliant.

TESTING INSTRUCTIONS

CI.

ADDITIONAL INFORMATION

• Has associated issue:
• 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

cc: @shawnzhu

[codecov]

Codecov Report

Merging #16040 (535e46d) into master (4cb79e5) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #16040      +/-   ##
==========================================
- Coverage   76.90%   76.90%   -0.01%     
==========================================
  Files         995      995              
  Lines       52842    52842              
  Branches     6709     6709              
==========================================
- Hits        40640    40637       -3     
- Misses      11976    11979       +3     
  Partials      226      226              

Flag	Coverage Δ
hive	81.32% <ø> (ø)
javascript	71.30% <ø> (-0.02%)	⬇️
mysql	81.58% <ø> (-0.05%)	⬇️
postgres	81.64% <ø> (ø)
presto	81.48% <ø> (ø)
python	82.16% <ø> (ø)
sqlite	81.28% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset-frontend/src/components/Select/Select.tsx	73.05% <0.00%> (-1.37%)	⬇️

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[graceguo-supercat]

LGTM