Fix: Double comma in `illegalClasses` property in `checkstyle.xml`

[nicolasgarea]

Description

Found a minor typo in the checkstyle.xml file: there's a double comma (, ,) in the value attribute for the illegalClasses property. This creates an empty entry in that comma-separated list of regular expressions.

To Reproduce

1. Navigate to the checkstyle.xml file in the main branch of the Spring Boot repository (src/checkstyle/checkstyle.xml).
2. Locate the <property name="illegalClasses" ...> element.
3. Observe the , , sequence within the value attribute.

Expected Behavior

The double comma should be removed, resulting in a clean, comma-separated list of illegalClasses without any empty entries.

Additional Context

While this typo is likely to be ignored by the Checkstyle tool itself and not cause functional errors in the build process, correcting it would improve the readability and consistency of the configuration file.

[snicoll]

@nicolasgarea we don't need an issue and a PR as one supersedes the other. If the intention is to submit a PR, please just do that.

closing in favor of PR #45975