QA: raise SciMLTesting compat floor to 1.7 for public-API EI gating

SciMLTesting 1.7 gates the two public-API ExplicitImports checks
(check_all_qualified_accesses_are_public / check_all_explicit_imports_are_public)
to Julia >= 1.11, which is the regime in which the stripped public-API
ignore lists are correct. Raising the floor from "1.6" to "1.7" ensures
CI resolves a SciMLTesting that runs those checks rather than potentially
an older release.

Verified against the registered releases (SciMLBase 3.27.0, DiffEqBase 7.6.0,
ModelingToolkit 11.29.0, Symbolics 7.29.0, SymbolicUtils 4.36.0,
SciMLTesting 1.7.0, ExplicitImports 1.15.0):

- Julia 1.12 (public-API checks run): emptying both public-API ignore lists
  leaves exactly two survivors flagged by all_qualified_accesses_are_public --
  topsort_equations (non-public in ModelingToolkit) and getname (re-exported
  by Symbolics from SymbolicIndexingInterface, non-public). Neither is a
  SciMLBase-owned name, so no DiffEqBase.X -> SciMLBase.X migration applies;
  both are restored. all_explicit_imports_are_public passes with an empty
  ignore list. QA group: 17/17 pass.
- Julia 1.10 (public-API checks skipped on the LTS): QA group 15/15 pass.

ModelOrderReduction's source makes no qualified SciMLBase/DiffEqBase accesses,
so steps 2 (caller-migrate) is a no-op for this package.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

test/qa/Project.toml

@@ -12,7 +12,7 @@ Aqua = "0.8"
 JET = "0.9, 0.10, 0.11"
 LinearAlgebra = "1"
 SafeTestsets = "0.0.1, 0.1"
-SciMLTesting = "1.6"
+SciMLTesting = "1.7"
 Test = "1"
 
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