Add warning ignore for StepAxisWarning from GWCS until WCSs are updated

[WilliamJamieson]

spacetelescope/gwcs#614 is introducing some changes that include a deprecation warning for the use of non-coordinate axes. JWST has several WCSs that make use of these. This PR is adding the warning ignore to avoid multiple test failures due to this warning until all the WCSs involved can have their coordinate frame metadata updated.

Tasks

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