Decrease backlight minimum for X10E on 2.9

[packetpilot]

If it looks familiar, it is! Same as #4539 but for the 2.9 branch.

Zero tears will be shed if this isn't something worth merging into the 2.9 branch.

I did build & flash this to my X10SE briefly, but decided running the actual 2.9.2 release w/ the same changes was a "safer" bet, so time spent was minimal, but so was the LCD backlight brightness, and zero flickers were observed. (I have a freakishly high flicker fusion threshold fwiw.)

[pfeerick]

PRs that not targeting main will generally not be accepted. The development line is main, and once a PR is merged, changes cherry-picked into the maintenance branches for earlier releases if deemed necessary. However given the amount of change involved, if the main version is accepted, this will go in also. Depending on how long that decision takes will determine if it makes it for 2.9.3 or not.

[ParkerEde]

I've tested it in 2.9.2. it works as expected