[Python][CI] Pin pandas version to 0.25 in the dask integration test

[asfimport]

Failing nightly build https://app.circleci.com/jobs/github/ursa-labs/crossbow/7693

Reporter: Krisztian Szucs / @kszucs
Assignee: Krisztian Szucs / @kszucs

PRs and other links:

• GitHub Pull Request #6326

_{Note: This issue was originally created as ARROW-7729. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 6326
#6326