Dandiset "modified" timestamp not updated when entries deleted from Zarr

When one or more entries are deleted from a Zarr via the `DELETE /zarr/{zarr_id}/files/` endpoint — even if the Zarr is then reingested by calling `POST /zarr/{zarr_id}/finalize/` — the "modified" properties of both the containing Dandiset and Dandiset version are not updated.

MVCE:

```python
import os
from pathlib import Path
from tempfile import TemporaryDirectory
from time import sleep
from dandi.consts import dandiset_metadata_file
from dandi.dandiapi import DandiAPIClient
from dandi.files import dandi_file
import numpy as np
import zarr

with DandiAPIClient.for_dandi_instance(
"dandi-staging", token=os.environ["DANDI_API_KEY"]
) as client:

d = client.create_dandiset(
"Test Zarr Dandiset",
{
"schemaKey": "Dandiset",
"name": "Test Dandiset",
"description": "A test Dandiset",
"contributor": [
{
"schemaKey": "Person",
"name": "Wodder, John",
"roleName": ["dcite:Author", "dcite:ContactPerson"],
}
],
"license": ["spdx:CC0-1.0"],
},
)

dandiset_id = d.identifier
print("DANDISET ID:", dandiset_id)
try:
with TemporaryDirectory() as tmpdir:
dspath = Path(tmpdir)
(dspath / dandiset_metadata_file).write_text(
f"identifier: '{dandiset_id}'\n"
)
zarr.save(dspath / "sample.zarr", np.arange(1000), np.arange(1000, 0, -1))
df = dandi_file(dspath / "sample.zarr", dandiset_path=dspath)
print("Uploading Zarr ...")
df.upload(d, {"path": "sample.zarr", "description": "A Zarr"})

d.refresh()
print("Dandiset modified at:", d.modified)
print("Dandiset draft version modified at:", d.version.modified)

print("Sleeping ...")
sleep(3)

(asset,) = d.get_assets()
entry = next(asset.iterfiles())
print("Deleting entry", entry, "from Zarr")
asset.rmfiles([entry])

d.refresh()
print("Dandiset modified at:", d.modified)
print("Dandiset draft version modified at:", d.version.modified)
finally:
print("Deleting Dandiset ...")
d.delete()
```

Sample output:

```text
DANDISET ID: 212987
Uploading Zarr ...
Dandiset modified at: 2024-02-26 14:06:12.328706+00:00
Dandiset draft version modified at: 2024-02-26 14:06:12.527791+00:00
Sleeping ...
Deleting entry .zgroup from Zarr
Dandiset modified at: 2024-02-26 14:06:12.328706+00:00
Dandiset draft version modified at: 2024-02-26 14:06:12.527791+00:00
Deleting Dandiset ...
```

Start by tracing the DELETE /zarr/{zarr_id}/files/ endpoint and POST /zarr/{zarr_id}/finalize/ entry points, using the supplied Python MVCE against dandi-staging. Confirm that deleting Zarr entries updates the containing Dandiset and Dandiset version modified timestamps, including after finalization.

Written by the indexing model from the issue text.