Some minor changes for supported versions of Checkstyle

[tsjensen]

Hi, I just noticed some major cleanup of the supported versions, and as you might expect, and I can't let the opportunity pass by without giving some comments.

TL/DR: We should make the changes in this PR. 😉

Long version:

• 6.12 → 6.12.1: probably ok, as people only see some warnings disappear
• 7.1, 7.1.1 → 7.1.2: probably ok too, as they tried to fix the equals/hashCode behavior. But that behavior does differ between versions.
• 7.6 → 7.6.1: I actually added back 7.6, because it threw different exceptions from the API, and this breaks custom checks
• 8.4 → 8.5: ✔
• 8.11, 8.12, 8.13 → 8.14: ✔
• I added 8.9 → 8.10.1
• plus, I added a mapping for 8.15 → 8.16, as these versions appear to be compatible, and 8.15 was missing so far.

Hope this helps!

[jshiell]

Magic - thank you! A second pair of eyes on this stuff is awesome, and I, as always, am much obliged.