Add a compile_error if setting selectable fields on K8s < 1.30

[clux]

As it is not available on older Kubernetes versions.

Doing this now causes:

error: selectable fields require Kubernetes >= 1.30
  --> examples/crd_derive.rs:12:10
   |
12 | #[derive(CustomResource, Serialize, Deserialize, Default, Debug, PartialEq, Eq, Clone, JsonSchema)]
   |          ^^^^^^^^^^^^^^
   |
   = note: this error originates in the derive macro `CustomResource` (in Nightly builds, run with -Z macro-backtrace for more info)

(can be seen if hacking examples/cargo.toml to use earliest from k8s-openapi)

Follow-up to #1610

[Danil-Grigorev]

Thank you, LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.3%. Comparing base (7526539) to head (48ec7a0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1612     +/-   ##
=======================================
+ Coverage   75.3%   75.3%   +0.1%     
=======================================
  Files         82      82             
  Lines       7340    7343      +3     
=======================================
+ Hits        5524    5527      +3     
  Misses      1816    1816             

Files with missing lines	Coverage Δ
kube-derive/src/custom_resource.rs	81.5% <100.0%> (+0.4%)	⬆️