feat: show progress of issues and PRs migrations (#12738)

These are by the far the longest time spent on during a migration.
Indicate the progress of how many issues and PRs were migrated so far.
Don't overwhelm the messenger, so they are only updated once a batch is
migrated. Which is "slow" enough to see it's not stuck and still doing work.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12738
Reviewed-by: Ellen Εμίλια Άννα Zscheile <fogti@noreply.codeberg.org>
Reviewed-by: 0ko <0ko@noreply.codeberg.org>

modules/migration/uploader.go

@@ -1,3 +1,4 @@
+// Copyright 2026 The Forgejo Authors. All rights reserved.
 // Copyright 2019 The Gitea Authors. All rights reserved.
 // Copyright 2018 Jonas Franz. All rights reserved.
 // SPDX-License-Identifier: MIT