Remove FeatureFlags#EXAMPLE

[AB-xdev]

FeatureFlags#EXAMPLE states:

Example feature. Will be removed once the first real feature flag is added

It should therefore be removed

flow/flow-server/src/main/java/com/vaadin/experimental/FeatureFlags.java

Lines 60 to 64 in ec83c9f

	public static final Feature EXAMPLE = new Feature(
	"Example feature. Will be removed once the first real feature flag is added",
	"exampleFeatureFlag", "https://github.com/vaadin/flow/pull/12004",
	false,
	"com.vaadin.flow.server.frontend.NodeTestComponents$ExampleExperimentalComponent");

[mcollovati]

As commented in the linked PR, removing the flag would negatively impact the quality of code base tests, because of workarounds that should be introduced to verify all functionalities related to feature flags.

I propose to preserve the feature, and update the description to state that it is there for testing purpose.

public static final Feature EXAMPLE = new Feature(
  "Example feature. Internally used for testing purposes. Does not have any effect on production applications.",
  "exampleFeatureFlag", "https://github.com/vaadin/flow/pull/12004",
  false,
  "com.vaadin.flow.server.frontend.NodeTestComponents$ExampleExperimentalComponent");