SingleGroupMatcher is chatty with wrong warning

### Affected version

3.5.3

### Bug description

When using `excludedGroups` to disable tagged test to run with junit5.
If the build is using junit4 as well for few modules of the same project.
Some noisy/wrong logs are generated such
```
[WARNING] Couldn't load group class 'external' in Surefire|Failsafe plugin. The group class is ignored!
[WARNING] Couldn't load group class 'large-disk-resource' in Surefire|Failsafe plugin. The group class is ignored!
[WARNING] Couldn't load group class 'stress' in Surefire|Failsafe plugin. The group class is ignored!
[WARNING] Couldn't load group class 'slow' in Surefire|Failsafe plugin. The group class is ignored!
[WARNING] Couldn't load group class 'flaky' in Surefire|Failsafe plugin. The group class is ignored!
```
As those values are tags used such `@Tag("flaky")` they should not be logged as `warning`. but `debug` only

Start by tracing SingleGroupMatcher with a mixed JUnit 4 and JUnit 5 build using excludedGroups. Reproduce the messages for values such as "flaky" and verify that tag-related group names are logged at debug rather than warning; the issue is complete when those warnings no longer appear.

Written by the indexing model from the issue text.