Deprecate IE related functions from FeatureDetection.js

[jfayot]

Support for IE was discontinued in Cesium 1.84

As a result, isInternetExplorer and other IE related functions from FeatureDetection.js can safely be deprecated.

See also PR #12581

[jfayot]

@ggetz , does it have to go through a deprecation phase ? and if so, how long ?

[ggetz]

It looks like isInternetExplorer is not a public function, and therefore does not require deprecation.