docs: remove outdated JUnit 4 dependency references from JUnit 5 page

[PreAgile]

Summary

Remove outdated references to JUnit 4.x dependency from the JUnit 5
integration documentation page.

Problem

The JUnit 5 docs page mentions that Testcontainers core depends on
JUnit 4.x, but this dependency was removed in #10805.
Two references are outdated:

• The opening sentence mentioning "tightly coupled with JUnit 4.x rule API"
• The Limitations section warning about JUnit 4.x ending up on the classpath

Fix

• Updated opening sentence to remove JUnit 4.x reference
• Removed the JUnit 4.x classpath paragraph from Limitations

Closes #11578

[eddumelendez]

Hi @PreAgile, with all due respect do not raise PRs with unrelated changes. Also, PR is out-dated and I started addressing those changes in previous commits.

Have you checked the contributing site? Before raising a PR, see if the issue has been triaged by maintainers.

Thanks.