fix(mssql): convert legacy DATETIME columns to DATETIME2 (#38216)

## Problem

On MSSQL databases created by old Gitea versions, the real datetime
columns `external_login_user.expires_at` and `lfs_lock.created` were
created as `DATETIME`. `DATETIME` parses datetime literals in a
locale-dependent way, so the ISO string `'YYYY-MM-DD HH:MM:SS'` that
xorm sends fails to convert when the session language is not English
(e.g. German defaults to `dmy`):

```
mssql: Bei der Konvertierung eines nvarchar-Datentyps in einen datetime-Datentyp liegt der Wert außerhalb des gültigen Bereichs.
```

This breaks linking an external (OAuth/Keycloak) account to an existing
user, and LFS lock creation, with a 500 error.

## Fix

Current xorm already maps `time.Time` to the locale-independent
`DATETIME2` for new installs, so only legacy databases are affected.
This adds migration `341` that converts these columns to `DATETIME2` on
legacy MSSQL databases (no-op on other databases and on columns already
using `DATETIME2`).

A full audit of persisted `time.Time` columns in `models/` confirmed
these two are the only real datetime columns affected — every other time
value is stored as a unix-timestamp integer.

A regression test (MSSQL-only, mirroring the existing v338 pattern)
downgrades the columns to legacy `DATETIME`, runs the migration, asserts
the type becomes `DATETIME2`, and verifies an ISO datetime insert
succeeds under `SET LANGUAGE German`.

Fixes #38211

models/migrations/migrations.go

@@ -418,6 +418,7 @@ func prepareMigrationTasks() []*migration {
 		newMigration(338, "Expand legacy MSSQL issue/comment long-text columns", v1_27.ExpandIssueAndCommentLongTextFieldsForMSSQL),
 		newMigration(339, "Extend action c_u index to include created_unix for faster dashboard feed queries", v1_27.AddCreatedUnixToActionUserIsDeletedIndex),
 		newMigration(340, "Add ContinueOnError column to ActionRunJob", v1_27.AddContinueOnErrorToActionRunJob),
+		newMigration(341, "Convert legacy MSSQL DATETIME columns to DATETIME2", v1_27.FixLegacyMSSQLDateTimeColumns),
 	}
 	return preparedMigrations
 }