Deprecate old user manager events for user creation

[MorrisJobke]

• add to Critical changes for developers & admins for Nextcloud 21 #23210

Found while looking into #23887

[faily-bot]

🤖 beep boop beep 🤖

Here are the logs for the failed build:

Status of 35032: failure

mariadb10.4-php7.4

Show full log

There were 2 warnings:
1) Test\Files\ViewTest::testRenameFailDeleteTargetKeepSource
Trying to configure method "writeStream" which cannot be configured because it does not exist, has not been specified, is final, or is static
2) Test\Files\ViewTest::testCopyFailDeleteTargetKeepSource
Trying to configure method "writeStream" which cannot be configured because it does not exist, has not been specified, is final, or is static
--
There was 1 failure:
1) Test\Share20\DefaultShareProviderTest::testGetSharesReshare
Failed asserting that '82' matches expected 0.
/drone/src/tests/lib/Share20/DefaultShareProviderTest.php:1423

[ChristophWurst]

Good call. #23768 should make it clear on the caller side