Run QPY backwards compat tests on trailing edge Python version

This commit moves the qpy backwards compatibility testing from the
leading edge python version, which in this PR branch is Python 3.11, to
the trailing edge Python version which is currently 3.7. Trying to add
support for a new Python version has demonstrated that we can't use the
leading edge version as historical versions of Qiskit used to generate
old QPY payloads are not going to be generally installable with newer
Python versions. So by using the trailing edge version instead we can
install all the older versions of Qiskit as there is Python
compatibility for those Qiskit versions. Eventually we will need to
raise the minimum Qiskit version we use in the QPY tests, when Python
3.9 goes EoL in October 2025 and Qiskit Terra 0.18.0 no longer has any
supported versions of Python it was released for. We probably could
get by another year until Python 3.10 goes EoL in 2026 it just means
we're building 0.18.x and 0.19.x from source for the testing, but when
Python 3.11 becomes our oldest supported version we'll likely have to
bump the minimum version.

This does go a bit counter to the intent of the test matrix to make the
first stage return fast and do a more through check in the second stage.
But, in this case the extra runtime is worth the longer term stability
in the tests.

azure-pipelines.yml

@@ -151,7 +151,7 @@ stages:
         - template: ".azure/test-linux.yml"
           parameters:
             pythonVersion: ${{ parameters.minimumPythonVersion }}
-            testQPY: false
+            testQPY: true
             testImages: false
             rustVersion: ${{ parameters.minimumRustVersion }}
 
@@ -171,7 +171,7 @@ stages:
         - template: ".azure/test-linux.yml"
           parameters:
             pythonVersion: ${{ parameters.maximumPythonVersion }}
-            testQPY: true
+            testQPY: false
             testImages: true
             installFromSdist: true