HW Relnotes: Add umb(4) modems to Misc Networks

The umb(4) MIBM cellular modem driver was added in src:0f1bf1c22a0c.
Add it to the Release Hardware Notes template for inclusion in
FreeBSD 15.

Reviewed by: adrian, mhorne
Accepted by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D48738