[fix] Update migration Revision for #6945

[graceguo-supercat]

SUMMARY

db migration in #6945 was based on an older head. I run superset db merge heads to make it compatible with master, but when we run superset db upgrade, somehow downgrade script from #6945 is not triggered.

Solution:
Now I rebased migration script to latest master to make sure revision is from latest master. So downgrade script will be executed.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

run superset db upgrade should see extra parents attribute in layout meta data.
run superset db downgrade should see parents attribute are removed.

ADDITIONAL INFORMATION

[ ] Has associated issue:
[ ] Changes UI
[ ] Requires DB Migration. Confirm DB Migration upgrade and downgrade tested.
[ ] Introduces new feature or API
[ ] Removes existing feature or API
[ ] Fixes bug
[ ] Refactors code
[ ] Adds test(s)

REVIEWERS

@michellethomas @john-bodley

[codecov-io]

Codecov Report

Merging #7258 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7258   +/-   ##
=======================================
  Coverage   64.72%   64.72%           
=======================================
  Files         425      425           
  Lines       20672    20672           
  Branches     2282     2282           
=======================================
  Hits        13380    13380           
  Misses       7167     7167           
  Partials      125      125

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[john-bodley]

I think you should keep the revision ID unchanged.

[graceguo-supercat]

after rebase, I created script from scratch, so the system generated new file name.

[graceguo-supercat]

thanks! i will test in local environment right after merge.