Azure SQL Managed Instance is bleeding edge. That is, migrating from SQL 2012 to Azure MI, is akin to upgrading to SQL 2019 and a little beyond that.
But you can put the database in compatibility mode 110 to have it as faithful as possible to SQL 2012.
Now, Microsoft are very conservative, so even if you go with compatibility level 150, it is not that likely that something will break in terms of functionality, but using the migration tool as mentioned by other is a good idea.
Performance is another thing, since there have been plenyt of changes to the optimizer. A good strategy is to stay on compat level 110 initially, and enable Query Store. After a week or two you flip the switch. If you get a plan regression, you can easily force the old plan a makeshift solution while solve the performance problem in a better way. There is tooling in SSMS to help you with this.