The Node portion of libmongocrypt's CI runs on the latest LTS Node version by default.  Come October 25th, Node's portion of the CI will run on Node18 (and no other Node version).  However, Node18 dropped support for glibc < 2.28, used on various distros in libmongocrypt's CI (ubuntu18 and rhel7 are two examples), resulting in CI failures when Node18 becomes LTS.

Additionally, mongodb-client-encryption is built during CI automatically and published from rhel7 variants.  We will need to either move CI to be published from a different variant, or ensure rhel7 stays on Node16.

AC:

• On systems that can support Node18, use it as the test runtime.
• Otherwise, fall back to Node16.
• Ensure that the pre-build publishing still happens in Rhel7, which only supports Node16.