Alerting: Increase size of kvstore value type for MySQL to LONGTEXT (#80331)

* Alerting: Increase size of kvstore value type for MySQL to LONGTEXT

alertmanager uses the kvstore to persist its notification log and the current
column limit for MySQL (16.7mb) puts the maximum entries at a level that is
potentially achievable for heavy alerting users (~40-80k entries).

In comparison, the current type for PSQL (TEXT) is effectively unlimited and
I believe SQLIte defaults to 2gb which is also plenty of leeway.

pkg/services/sqlstore/migrations/migrations.go

@@ -113,6 +113,8 @@ func (*OSSMigrations) AddMigration(mg *Migrator) {
 	ssosettings.AddMigration(mg)
 
 	ualert.CreateOrgMigratedKVStoreEntries(mg)
+
+	addKVStoreMySQLValueTypeLongTextMigration(mg)
 }
 
 func addStarMigrations(mg *Migrator) {