"On topic": I know it is too late for the original poster (https://moodle.org/mod/forum/discuss.php?d=353276&parent=1472847) but I am surprised that nobody has suggested the LTS-route, -> 3.1 -> 3.5 - https://docs.moodle.org/dev/Releases#Version_support. (Well, Dan did, without mentioning LTS: https://moodle.org/mod/forum/discuss.php?d=353276#p1425425.)
Off-topc: Background of the utf8mb4 confusion: https://moodle.org/mod/forum/discuss.php?d=363207#p1464839
Installing and upgrading help
Upgrade to Moodle 3.2 or 3.3?
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