Update default room to version 6

[clokep]

Synapse v1.14.0 included support for room version 6 (see #7500). At some point we'll want to make this the default once people have had time to upgrade to compatible homeservers.

[TheEvilSkeleton]

Just a suggestion, I personally think that it would be a better idea to fix the issues before making it the default:

element-hq/element-android#1558
element-hq/element-android#1879

[TheEvilSkeleton]

Awesome :) element-hq/element-android#2010

[turt2live]

MSC: matrix-org/matrix-spec-proposals#2788

[aaronraimist]

Fixed by #8461