ACM dropped prefix "DOI:"

[fgnievinski]

ACM recently updated their Citation Style and Reference Formats webpage:
https://www.acm.org/publications/authors/reference-formatting

They've simplified the formatting of DOIs; previously it was, e.g.:
DOI:https://doi.org/10.1145/1188913.1188915
Now it's just:
https://doi.org/10.1145/1188913.1188915

I've confirmed it via email at production@hq.acm.org

I've updated only the two ACM SIG CSLs:
#6600
#6599

However, there are a lot of other "ACM" CSLs, pending:
https://editor.citationstyles.org/searchByName/?style=acm

I've focused on the ones used by default in PKP/OJS, e.g.:
https://demo.publicknowledgeproject.org/ojs3/demo/index.php/demojournal/article/view/6

Previous related issue:

• ACM SIG should include DOI #1996

PS: the complete prefix implementation in one of the two CSLs updated used to contain a full period separator: . DOI:https://doi.org/ -- I've removed the period since it didn't seem to make a difference in the Visual Editor, please confirm.
https://editor.citationstyles.org/styleInfo/?styleId=http%3A%2F%2Fwww.zotero.org%2Fstyles%2Facm-sig-proceedings
https://editor.citationstyles.org/styleInfo/?styleId=http%3A%2F%2Fwww.zotero.org%2Fstyles%2Facm-sig-proceedings-long-author-list