Links to older versions of Checkstyle release notes should have simple direct link

From https://github.com/checkstyle/checkstyle/pull/11749/files#r901140140:
>`https://checkstyle.sourceforge.io/version/6.18/: 403 Forbidden`

failure is stable, several executions fails in row.

on section:
```xml

Current site contains documentation only for latest, released version.
We only support this latest version. We highly recommend read latest documentation first.


However, you might need documentation for previous releases because you are still using
an older version and latest version describe some features that does not work for you.


You can find documentation for most old versions using a URL format like
https://checkstyle.sourceforge.io/version/X.X where "X.X" is the version number.

Example:

https://checkstyle.sourceforge.io/version/6.18

for version 6.18.

```
Old link from current documentation about release notes:

https://checkstyle.sourceforge.io/version/6.18

New link to current checkstyle.org site:

https://checkstyle.org/releasenotes_old_6-0_7-8.html#Release_6.18

In previous documentation about finding release notes of older versions, it was easy to understand ` https://checkstyle.sourceforge.io/version/X.X where "X.X" is the version number.`

Now, we would need to say something like: `https://checkstyle.org/releasenotes_old__.html#Release_, where starting and ending release must be hyphenated major-minor version, in one of two ranges (if your version is less than 7.9): 6-0_7-8 OR 5.9_1.0, and actual release number is in the form of major.minor.patch (if patch is applicable).`

This is complicated and more confusing than just scrolling to the bottom of the release notes and clicking on the link to older versions, see right below https://checkstyle.org/releasenotes.html#Release_8.0

We should either completely remove this entire `Previous Version Documentation` section, or overhaul how we directly link to older releases.

Start by locating the “Previous Version Documentation” section and reviewing the current release-notes page around the older-version links mentioned in the issue. Compare the confusing direct-link instructions with the navigation below Release 8.0; done means the section is either removed or provides a clear, working way to reach older release notes.

Written by the indexing model from the issue text.