[v2] Upgrading to 2.8 prevents clean downgrade path

__Steps to reproduce:__
List the minimal actions needed to reproduce the behaviour.

1. Work with 2.7
2. Upgrade to 2.8
3. Downgrade back to 2.7

__Expected behaviour:__
It should maintain a compatible DB and not break it.

__Actual behaviour:__
Downgrade fails.

__Environment info:__

Running on docker. I cannot run `influxd downgrade` because I just switch between images.

__Logs:__
```
E 2026-04-26T09:40:14.692494180Z info found existing boltdb file, skipping setup wrapper {"system": "docker", "bolt_path": "/var/lib/influxdb2/influxd.bolt"}
E 2026-04-26T09:40:14.748595353Z info found existing boltdb file, skipping setup wrapper {"system": "docker", "bolt_path": "/var/lib/influxdb2/influxd.bolt"}
I ts=2026-04-26T09:40:14.932983Z lvl=info msg="Welcome to InfluxDB" log_id=12UIQRq0000 version=v2.7.12+SNAPSHOT.6584e05ad3 commit=6584e05ad3 build_date=2026-03-23T08:54:05Z log_level=info
W ts=2026-04-26T09:40:14.933131Z lvl=warn msg="nats-port argument is deprecated and unused" log_id=12UIQRq0000
I ts=2026-04-26T09:40:14.935727Z lvl=info msg="Resources opened" log_id=12UIQRq0000 service=bolt path=/var/lib/influxdb2/influxd.bolt
I ts=2026-04-26T09:40:14.936097Z lvl=info msg="Resources opened" log_id=12UIQRq0000 service=sqlite path=/var/lib/influxdb2/influxd.sqlite
E ts=2026-04-26T09:40:14.938000Z lvl=error msg="Failed to apply KV migrations" log_id=12UIQRq0000 error="up: reading migrations: DB contains record of unknown migration \"create authorizationhashedindexv1 bucket\" - if you are downgrading from a more recent version of influxdb, please run the \"influxd downgrade\" command from that version to revert your metadata to be compatible with this version prior to starting influxd."
E Error: up: reading migrations: DB contains record of unknown migration "create authorizationhashedindexv1 bucket" - if you are downgrading from a more recent version of influxdb, please run the "influxd downgrade" command from that version to revert your metadata to be compatible with this version prior to starting influxd.
E See 'influxd -h' for help
```

Start by reproducing the upgrade and downgrade sequence with the 2.7 and 2.8 Docker images, using the reported influxd migration error as the entry point. Investigate how the authorizationhashedindexv1 migration is recorded and handled; done means a database upgraded to 2.8 can return to 2.7 without breaking compatibility.

Written by the indexing model from the issue text.