Add migration to upgrade relations and texts to v2

[jotare]

Description

Describe the proposed changes made in this PR.

How was this PR tested?

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[shortcut-integration]

This pull request has been linked to Shortcut Story #8057: Script/migration to remove old relations index.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (d487a11) 79.40% compared to head (0c434f6) 82.24%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
...grations/0007_upgrade_relations_and_texts_to_v2.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1732      +/-   ##
==========================================
+ Coverage   79.40%   82.24%   +2.84%     
==========================================
  Files         334      335       +1     
  Lines       19550    19738     +188     
==========================================
+ Hits        15523    16233     +710     
+ Misses       4027     3505     -522     

Flag	Coverage Δ
ingest	69.05% <ø> (-0.09%)	⬇️
nucliadb	70.40% <80.00%> (+4.71%)	⬆️
reader	79.02% <ø> (ø)
sdk	40.58% <ø> (ø)
train	63.71% <ø> (-0.17%)	⬇️
utils	81.25% <ø> (ø)
writer	85.15% <ø> (+0.69%)	⬆️

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