Address deprecations: remove _param_watchers, raise RuntimeError on unsafe ops during init, and failed validation of a parameter default after inheritance

[maximlt]

This PR touches these deprecated features that are now emitted at the FutureWarning level:

Warning	Description	Action
ParamFutureWarning since 2.0.0	Parameterized namespace / instance._param_watchers (getter and setter): use instead the property inst.param.watchers	Removed
ParamFutureWarning since 2.0.0	Warn on failed validation of the default value of a Parameter after the inheritance mechanism has completed	raises RuntimeError
ParamFutureWarning since 2.0.0	Running unsafe instance.param.objects(instance=True) during Parameterized instance initialization, instead run it after having called super().__init__(**params)	raises RuntimeError
ParamFutureWarning since 2.0.0	Running unsafe instance.param.trigger(<>) during Parameterized instance initialization, instead run it after having called super().__init__(**params)	raises RuntimeError
ParamFutureWarning since 2.0.0	Running unsafe instance.param.watch(callback, "<param_name>") during Parameterized instance initialization, instead run it after having called super().__init__(**params)	raises RuntimeError

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.22%. Comparing base (71e8e47) to head (289bbc6).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #973      +/-   ##
==========================================
- Coverage   87.26%   87.22%   -0.05%     
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  Files           9        9              
  Lines        4948     4938      -10     
==========================================
- Hits         4318     4307      -11     
- Misses        630      631       +1     

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[hoxbro]

LGTM.

I can see we in panel have some compatibility code around _param_watchers. This compatibility code could be removed, right?

[maximlt]

I can see we in panel have some compatibility code around _param_watchers. This compatibility code could be removed, right?

holoviz/panel#7527