Fixes: #8901
Provide a convenient API `MockIntegrationContext.substituteTriggerFor(String pollingAdapterId, Trigger trigger)`
to mitigate the time span which might be provided in the original configuration
* Add `MockIntegrationContext.substituteTriggerFor(String pollingAdapterId, Trigger trigger)`
* Refactor `MockIntegrationContext` for Java 17 code style
* Change `MockIntegrationContext.beans` to `MultiValueMap` since we may replace several properties of
the same endpoint
* Modify `MockMessageSourceTests.testMockMessageSource()` to use new `substituteTriggerFor()`, too
* Add Javadoc for the `OnlyOnceTrigger`
* Document the new feature and mention it with correlation to `OnlyOnceTrigger`
1d73ab59af