Change internal code names of postingsFormats to use 84 suffix

Some postings formats write the postings differently as of Lucene 8.4 due to changes – LUCENE-9027 and LUCENE-9116. Blocktree was transitioned in a backwards-compatible way but some (all?) others were not. Consequently an attempt of the new version to read an old index will fail due to some non-obvious error. I propose here using a simple version suffix on these postings formats like "84" (thus "FST84" as one example). I see some already use a suffix but were not bumped for 8.4. This is a really simple change and doesn't address the problem of us not noticing future needs to version bump.