releases/15.0R/relnotes: Use 13.0R formatting for "Sponsored by"

We have been having some special formatting for "Sponsored by", which
for some reason has been abandoned since 14.0R.

Put it back, as it helps distinguish the content from the sponsoring.
It is also visually more pleasing (YMMV).

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53985