doc: add release notes for 8.21
doc: add release notes for 8.21

romani/checkstyle,checkstyle/checkstyle,romani/checkstyle,checkstyle/checkstyle,rnveach/checkstyle,rnveach/checkstyle,romani/checkstyle,checkstyle/checkstyle,rnveach/checkstyle,rnveach/checkstyle,romani/checkstyle,checkstyle/checkstyle,romani/checkstyle,romani/checkstyle,checkstyle/checkstyle,rnveach/checkstyle,checkstyle/checkstyle,rnveach/checkstyle