With a deprecated Os, you can't update to minor release if a major release is available

[Stefal]

Describe the bug
With Debian 11 and RTKBase 2.6.1, I can't update to RTKBase 2.6.4

Expected behavior
We should be able to update to v2.6.4

Additional context
After investigation for #518 I noticed that when checking the lastest available release, the rtkbase_update.sh script test the operating system release at first, then stop the script because the release 2.7.0 needs Debian 12 or more.
We can't just run the upd_2.6.0() to upd_2.6.3() functions because it will use the content from the 2.7.0 release.
We should find a way to get the previous archive and use its content and update script.