Allow Stream<? extends TestTemplateInvocationContext> return types

[marcphilipp]

For consistency with ClassTemplateInvocationContextProvider and to
make implementations simpler, this commit changes the return type of the
provideTestTemplateInvocationContexts(ExtensionContext) method of the
TestTemplateInvocationContextProvider interface to allow more concrete
Streams to be returned.

This change is source compatible, meaning implementations will still
compile when declaring Stream<TestTemplateInvocationContext> as method
return type. To check for binary compatibility, I used Pioneer's
RetryingTestExtension with a 6.0.0-SNAPSHOT that included this change.
The extension kept working. Nevertheless, I included this change as
potentially breaking in the release notes.

Resolves #3577.

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes