Avoid losing token when updating mirror settings (#30429) (#30464)

Avoid losing token when updating mirror settings (#30429) (#30464)

Backport #30429 by @wolfogre

Fix #30416.

Before (it shows as "Unset" while there's a token):

<img width="980" alt="image"
src="https://github.com/go-gitea/gitea/assets/9418365/d7148e3e-62c9-4d2e-942d-3d795b79515a">

After:

<img width="977" alt="image"
src="https://github.com/go-gitea/gitea/assets/9418365/24aaa1db-5baa-4204-9081-470b15ea72b5">

The username shows as "oauth2" because of
https://github.com/go-gitea/gitea/blob/f9fdac9809335729b2ac3227b2a5f71a62fc64ad/services/migrations/dump.go#L99

I have checked that all usage of `MirrorRemoteAddress` has been updated.

<img width="1806" alt="image"
src="https://github.com/go-gitea/gitea/assets/9418365/2f042501-2824-4511-9203-c84a6731a02d">

However, it needs to be checked again when backporting.

Co-authored-by: Jason Song <i@wolfogre.com>