Remove Python<3.14 pin

[pp-mo]

Introduced by #6773 (but will soon need to be updated in the conda feedstock also) Relevant issue links:

• https://github.com/SciTools/iris/issues/6762
• parallel problem in matplotlib : https://github.com/matplotlib/matplotlib/issues/29185
  • N.B. this indicates that we may need to wait to remove, until fixed in all our dependencies

[rcomer]

The Matplotlib problem was fixed in v3.10.5 by https://github.com/matplotlib/matplotlib/pull/30198