This commit updates our platform support matrix to reflect upcoming
changes. The first is that in Rust 1.74 the Rust programming language is
raising their minimum support macOS version to 10.12, so Qiskit is
raising it's supported version of macOS to match this. The second change
is making symengine a hard requirement. We previously had symengine as a
requirement only on platforms that had precompiled packages available.
But, the percentage of our user base that runs qiskit on those platforms
is very small, and maintaining dual support for symengine and sympy adds
a lot of complexity around managing the dependencies. This commit
promotes symengine to a hard requirement for all users regardless of
platform. As a result Linux i686 and 32 bit Windows for Python < 3.10
has been downgraded to tier 3 support as you'll need a C++ to install
Qiskit on that platform now (regardless of Python version).

Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>