Restore support for ^8 || ^9

Problem/Motivation

This module used to support ^8 || ^9 || ^10. Recent commits bump this to ^10.1 as the minimum version. There's no major reason why that's needed, only to get the tests passing. A little bit of jumping through hoops in DurationFieldBrowserTestBase would allow this to keep working with a wider range of core versions.

Steps to reproduce

Proposed resolution

Add another conditional check on \Drupal::VERSION in DurationFieldBrowserTestBase.
Re-expand core_version_requirement

Remaining tasks

User interface changes

API changes

Data model changes