Update README to exclude Python 3.12 from supported versions.

[nden]

This PR adds caveats back into the README, warning users that they need to create an environment using python < 3.12

Checklist for maintainers

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• Make sure the JIRA ticket is resolved properly

[tapastro]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4fb9f14) 75.25% compared to head (db96e00) 75.25%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8161   +/-   ##
=======================================
  Coverage   75.25%   75.25%           
=======================================
  Files         470      470           
  Lines       38426    38426           
=======================================
  Hits        28916    28916           
  Misses       9510     9510           

Flag	Coverage Δ		*Carryforward flag
nightly	77.37% <ø> (ø)		Carriedforward from 4fb9f14

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