Update announce email template

See https://git.glasklar.is/system-transparency/core/stboot/-/blob/main/docs/cheat-sheet-on-releases.md?ref_type=heads#announcement-email-template, seems like it's lagging behind the almost similar but updated https://lists.system-transparency.org/mailman3/hyperkitty/list/st-announce@lists.system-transparency.org/thread/332SWEPTXANDOSPIDTWXDTY4MCHTT2LD/.