DAOS-18991 test: add mpich to requirements-ftest.txt

[daltonbohning]

Add mpich to requirements-ftest.txt so mpi4py can work with mpich.

Test-tag: LlnlMpi4py IorSmall mpich
Skip-unit-tests: true
Skip-fault-injection-test: true

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Ticket title is 'mpiio/llnl_mpi4py.py:LlnlMpi4py.test_mpi4py - libmpi.so: cannot open shared object file: No such file or directory on Leap15.6'
Status is 'In Progress'
Labels: 'ci_master_daily'
https://daosio.atlassian.net/browse/DAOS-18991

[daosbuild3]

Test stage Functional on EL 9 completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-18336/1/testReport/