Recommend/convert to use case-sensitive collation for MySQL/MSSQL (#28662)

Mainly for MySQL/MSSQL.

It is important for Gitea to use case-sensitive database charset
collation. If the database is using a case-insensitive collation, Gitea
will show startup error/warning messages, and show the errors/warnings
on the admin panel's Self-Check page.

Make `gitea doctor convert` work for MySQL to convert the collations of
database & tables & columns.

* Fix #28131

## ⚠️ BREAKING ⚠️

It is not quite breaking, but it's highly recommended to convert the
database&table&column to a consistent and case-sensitive collation.

routers/web/web.go

@@ -678,6 +678,8 @@ func registerRoutes(m *web.Route) {
 		m.Get("", admin.Dashboard)
 		m.Post("", web.Bind(forms.AdminDashboardForm{}), admin.DashboardPost)
 
+		m.Get("/self_check", admin.SelfCheck)
+
 		m.Group("/config", func() {
 			m.Get("", admin.Config)
 			m.Post("", admin.ChangeConfig)