fix: improve performance of recent `SlackChannel` related migrations

[joeyorlando]

What this PR does

After deploying r439-v1.12.0 to staging, I noticed that the migrations were taking a long time, and caused some wonkiness (see https://raintank-corp.slack.com/archives/C08063QES5N).

Apply all migrations: [redacted secret grafana-admin-creds:admin-user], alerts, auth, auth_token, base, contenttypes, email, exotel, fcm_django, google, heartbeat, labels, mobile_app, oss_installation, phone_notifications, schedules, sessions, slack, social_django, telegram, twilioapp, user_management, webhooks, zvonok
Running migrations:

source=engine:app google_trace_id=none logger=apps.alerts.migrations.0063_migrate_channelfilter_slack_channel_id Starting migration to populate slack_channel field.
source=engine:app google_trace_id=none logger=apps.alerts.migrations.0063_migrate_channelfilter_slack_channel_id Bulk updated 1 ChannelFilters with their Slack channel.
source=engine:app google_trace_id=none logger=apps.alerts.migrations.0063_migrate_channelfilter_slack_channel_id Finished migration to populate slack_channel field.
  Applying alerts.0063_migrate_channelfilter_slack_channel_id... OK

source=engine:app google_trace_id=none logger=apps.alerts.migrations.0064_migrate_resolutionnoteslackmessage_slack_channel_id Starting migration to populate slack_channel field.
source=engine:app google_trace_id=none logger=apps.alerts.migrations.0064_migrate_resolutionnoteslackmessage_slack_channel_id Bulk updated 1 ResolutionNoteSlackMessage records with their Slack channel.
source=engine:app google_trace_id=none logger=apps.alerts.migrations.0064_migrate_resolutionnoteslackmessage_slack_channel_id Finished migration to populate slack_channel field.
  Applying alerts.0064_migrate_resolutionnoteslackmessage_slack_channel_id... OK

source=engine:app google_trace_id=none logger=apps.schedules.migrations.0019_auto_20241021_1735 Starting migration to populate slack_channel field.
source=engine:app google_trace_id=none logger=apps.schedules.migrations.0019_auto_20241021_1735 Bulk updated 6 OnCallSchedules with their Slack channel.
source=engine:app google_trace_id=none logger=apps.schedules.migrations.0019_auto_20241021_1735 Finished migration to populate slack_channel field.
  Applying schedules.0019_auto_20241021_1735... OK

source=engine:app google_trace_id=none logger=apps.user_management.migrations.0026_auto_20241017_1919 Starting migration to populate default_slack_channel field.
source=engine:app google_trace_id=none logger=apps.user_management.migrations.0026_auto_20241017_1919 Bulk updated 1 organizations with their default Slack channel.
source=engine:app google_trace_id=none logger=apps.user_management.migrations.0026_auto_20241017_1919 Finished migration to populate default_slack_channel field.
  Applying user_management.0026_auto_20241017_1919... OK

NOTE: wrt these migrations already being run for certain OSS stacks; it shouldn't have much of an impact on OSS deployments, as it's really only an issue for very large versions of these tables (particularly the ResolutionNoteSlackMessage table, which by its nature, has a tendency to generate a lot of data).

Checklist

• [ ] Unit, integration, and e2e (if applicable) tests updated
• [ ] Documentation added (or pr:no public docs PR label added if not required)
• [ ] Added the relevant release notes label (see labels prefixed w/ release:). These labels dictate how your PR will show up in the autogenerated release notes.

[bpedersen2]

The SQL is mysql-specifc, see #5244