standardize requirements

[bjlittle]

This PR rationalizes and standardizes the iris-esmf-regrid requirements structure i.e.,

• remove the requirements/ci directory
• move the requirements/ci/nox.lock directory to requirements/locks
• update connective tissue from these changes within iris codebase

A subsequent PR will target SciTools/workflows to align the refresh-lockfiles.yml GHA with this structure.

Reference:

• https://github.com/SciTools/workflows/pull/8
• https://github.com/SciTools/iris/pull/5204
• https://github.com/SciTools/iris-grib/pull/327

[codecov[bot]]

Codecov Report

Merging #244 (10c5ec1) into main (b22062d) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #244   +/-   ##
=======================================
  Coverage   99.22%   99.22%           
=======================================
  Files          28       28           
  Lines        3102     3102           
=======================================
  Hits         3078     3078           
  Misses         24       24           

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[bjlittle]

@trexfeathers Awesome, cheers buddy :beers: