chore: Deprecate all classes in testbench-unit modules

[Artur-]

Mark all public classes, interfaces, enums, and annotations in the vaadin-testbench-unit-* modules as @deprecated(forRemoval = true) with a message directing users to migrate to Vaadin Browserless Test.

No implementation changes — only deprecation annotations and Javadoc @deprecated tags added. The existing code continues to work unchanged.

[ZheSun88]

maybe adding this message to the old artifacts is also useful

<project>
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.old.group</groupId>
    <artifactId>old-artifact</artifactId>
    <version>1.0.1</version>
    <packaging>pom</packaging>

    <distributionManagement>
        <relocation>
            <groupId>com.new.group</groupId>
            <artifactId>new-artifact</artifactId>
            <version>2.0.0</version>
            <message>This artifact has moved to com.new.group:new-artifact</message>
        </relocation>
    </distributionManagement>
</project>

[github-actions]

Test Results

 22 files   22 suites   21m 24s ⏱️
 83 tests  80 ✅ 3 💤 0 ❌
166 runs  160 ✅ 6 💤 0 ❌

Results for commit 73b2060.

♻️ This comment has been updated with latest results.

[Artur-]

now there is relocation info but that logs the message 293 times in the validation build..

[mcollovati]

The relocation message seems a bit incomplete: just changing packages is not enough to migrate since also base tests classes had names changed.
And JUnit 4 test cannot be migrated without JUnit 6 migration.
Maybe put a link to the new migration guide in there?

[mcollovati]

Is it so that relocation automatically downloads and replaces the artifact with the new one?
If so, this means that the existing project will break immediately.

[mcollovati]

I propose to remove the relocation stuff, since this is not a drop-in without side effects.