Bump k8s-openapi to 0.23 for Kubernetes 1.31 support

[clux]

Bumps to k8s-openapi 0.23 for Kubernetes v1_31 support.
As per policy, also bumps MK8SV to 1.26.

Note:

• minor internal change in client_ext due to the type change in LocalObjectReference: before / after.
• removal of 1.25 gate as 1.26 is MSRV so Ephemeral containers are GA for us
• cargo-msrv has some issues with new workspace layout - local check run gave a bump to 1.77.2 so have committed that

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.3%. Comparing base (6118818) to head (f51471f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1581   +/-   ##
=====================================
  Coverage   75.3%   75.3%           
=====================================
  Files         82      82           
  Lines       7331    7331           
=====================================
  Hits        5519    5519           
  Misses      1812    1812           

Files with missing lines	Coverage Δ
kube-client/src/api/mod.rs	68.1% <ø> (ø)
kube-client/src/client/client_ext.rs	84.5% <100.0%> (ø)