Raise ArrayInterface compat floor to 7.1 (fix downgrade CI)

[ChrisRackauckas-Claude]

Problem

The downgrade / Downgrade Tests check on main is red. The downgrade job pins ArrayInterface to its compat floor (7.0.0). At 7.0.0, ArrayInterface.fast_scalar_indexing(::JLArray) returns true because the GPUArraysCore-aware definition that correctly reports false for GPU-backed arrays was only added in ArrayInterface 7.1.0.

As a result, the _require_fast_scalar_indexing guard never fires on the downgrade floor: the package proceeds to scalar-index the GPU array, and GPUArrays throws a plain ErrorException ("Scalar indexing is disallowed.") instead of the package's intended ArgumentError. This breaks the JLArray error messages testset in test/interface_compatibility_tests.jl, which asserts an ArgumentError mentioning "fast scalar indexing" / "GPU arrays".

Fix

Raise the ArrayInterface compat floor from "7" to "7.1". The package's documented behavior (rejecting GPU arrays with a clear ArgumentError) only holds from ArrayInterface 7.1.0 onward, so 7.1 is the correct minimum bound.

Verification (local, Julia 1.10.11)

Bisected ArrayInterface against JLArrays 0.1.1 (the downgrade floor for JLArrays):

7.0.0 => fast_scalar_indexing=true   (guard never fires -> ErrorException, test fails)
7.1.0 => fast_scalar_indexing=false  (guard fires -> ArgumentError, test passes)

With ArrayInterface 7.1.0 + JLArrays 0.1.1, all 8 assertions in the "JLArray error messages" testset pass:

[ Info: ArrayInterface=7.1.0 JLArrays=0.1.1 fast_scalar_indexing=false
Test Summary:          | Pass  Total  Time
JLArray error messages |    8      8  0.1s

Please ignore until reviewed by @ChrisRackauckas.