Cutout bounds fix

[at88mph]

Changes

• NDimensionalSlicer: Fills cornet/span logic when a cutout’s requested bounds extend past the real image, using intersection so lengths and striding are correct instead of capping the difference in a way that can mis-size slices.
• EnergyCutout: Clamps the requested barycentric metre interval to the cube’s spectral extent (linear WCS at channels 1 and N before WCS sky2pix, so e.g. (-Inf, λ) past the high edge still clips to the band instead of no overlap; overlap in pixel space uses a 1…N native span consistent with FITSCutout.clip.
• AxisBoundsFiller (replaces the former “bounds clipper” idea): Puts the clipped 1D interval on the chosen energy/polarization axis and [1, NAXISn] on all other axes (no longer reusing a single “length 60” for every axis).
• PolarizationCutout: Returns null when there is no overlap on the Stokes axis (instead of an empty long[]); uses AxisBoundsFiller with per-axis sizes.
• Gradle upgrade for version 8 to allow for JDK 21, and added unit tests

Use case was to fix an Energy cutout where requested bounds exceeded the image cube bounds, but should be clipped.

[pdowler]

I do not want to upgrade java version and gradle wrapper in this repo yet... need to evaluate impact on other devs.

[pdowler]

Some details that should be looked at carefully.