Move cplex and docplex to pypi-only installs

[mrmundt]

Fixes ... our infrastructure

Summary/Motivation:

Something has been up on our linux/conda build for a minute, attributed to a new release on conda. Per Cplex official docs: "The CPLEX and CP Optimizer Python interfaces are no longer delivered inside CPLEX Optimization Studio, but can be installed directly from PyPI." Also... They are also moving away from conda and will remove support in a future release. So time to rip off the bandaid.

Changes proposed in this PR:

• Move cplex and docplex to be pypi-only installs.

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[blnicho]

We should also update the solver installation table (https://pyomo.readthedocs.io/en/stable/getting_started/solvers.html) to discourage users from trying to install CPLEX with conda.

[jsiirola]

One minor change in scoping, then this should be OK

[mrmundt]

We should also update the solver installation table (https://pyomo.readthedocs.io/en/stable/getting_started/solvers.html) to discourage users from trying to install CPLEX with conda.

Done!