website: Update minimum Clang version to 6

Due to changes here:
https://gem5-review.googlesource.com/c/public/gem5/+/45621,
the minimum supported Clang version is now 6.

Change-Id: I81c5215c3696225990ecb0cf185aec93d9c75c35
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/45699
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>