[migration] drop identity_relation if it exists It has been generated erroneously for a while, then fixed but not properly deleted from old databases. Don't use IF EXISTS for sqlserver compat.

Release notes
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.. toctree::
    :maxdepth: 1

    3.22
    3.21
    3.20
    3.19
    3.18
    3.17
    3.16
    3.15
    3.14