fix(core): remove duplicate MySQL user prefix

[MichaIng]

• Resolves: #

Summary

PR #53212 implemented a hardcoded database username, instead of deriving it from the Nextcloud admin username, to be able to setup Nextcloud without admin user.

However, it used oc_admin, while the MySQL database setup adds oc_ again in the dedicated createSpecificUser() function, resulting in oc_oc_admin. In case of PostgreSQL, this was done in setupDatabase(), replaced with the hardcoded username.

I kept the 16 characters limit, not sure whether this is still needed. The function is actually called only internally, only in this very script, so it should never see any other value than oc_admin. Let me know if I should remove the 0-16 substring, along with the confusing $adminUser variable (since this user is neither the database admin, nor the Nextcloud admin).

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

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[joshtrichards]

This one (among others) should already be addressed in #59094. Since I've been doing various refactoring in that area of code already, I'd obviously prefer to go that route... ;-)

[MichaIng]

@joshtrichards Nope, your PR does not address this, it still adds oc_ to the name in createSpecificUser, while that is called with a hardcoded oc_admin input already. But your PR clarifies the 16 characters limit, and makes the fix easier. I'll do the suggestion there, but can also rebase after your PR has been merged.