chore: add DASHBOARD_FILTERS_EXPERIMENTAL ff to BE default value

[amitmiran137]

SUMMARY

this flag was introduced in the frontend but default value is missing in the BE
#14814

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

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

[codecov]

Codecov Report

Merging #15417 (b2332b9) into master (f53b2c6) will decrease coverage by 0.22%.
The diff coverage is n/a.

❗ Current head b2332b9 differs from pull request most recent head 8fe89ec. Consider uploading reports for the commit 8fe89ec to get more accurate results

@@            Coverage Diff             @@
##           master   #15417      +/-   ##
==========================================
- Coverage   77.22%   76.99%   -0.23%     
==========================================
  Files         975      975              
  Lines       50634    50634              
  Branches     6222     6222              
==========================================
- Hits        39104    38988     -116     
- Misses      11319    11435     +116     
  Partials      211      211              

Flag	Coverage Δ
hive	?
mysql	81.71% <ø> (ø)
postgres	81.73% <ø> (ø)
presto	?
python	81.81% <ø> (-0.44%)	⬇️
sqlite	81.35% <ø> (ø)

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

Impacted Files	Coverage Δ
superset/config.py	91.15% <ø> (ø)
superset/db_engines/hive.py	0.00% <0.00%> (-82.15%)	⬇️
superset/db_engine_specs/hive.py	69.20% <0.00%> (-17.21%)	⬇️
superset/db_engine_specs/presto.py	83.36% <0.00%> (-6.53%)	⬇️
superset/views/database/mixins.py	81.03% <0.00%> (-1.73%)	⬇️
superset/connectors/sqla/models.py	88.22% <0.00%> (-1.65%)	⬇️
superset/db_engine_specs/base.py	87.97% <0.00%> (-0.41%)	⬇️
superset/models/core.py	89.76% <0.00%> (-0.27%)	⬇️
superset/utils/core.py	88.97% <0.00%> (-0.13%)	⬇️

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