CHANGELOG: no em dashes, semicolons or mid-sentence colons (#504)

Release headings use " - " before the date; changelog.astro,
site/scripts/changelog-dates.mjs and scripts/release-notes.mjs all accept
either. Prose rewritten with the smallest change that removes the mark;
sentences without one are untouched, and colons that end a line before a
list, or sit inside code and URLs, stay.

Author: fstubner