[BUG] astype ignores time zone

[MarcoGorelli]

Describe the bug
A clear and concise description of what the bug is.

Steps/Code to reproduce bug

import cudf
from datetime import datetime, timezone

s = cudf.Series([datetime(2020, 1, 1, tzinfo=timezone.utc)])
s.astype('datetime64[ns, Asia/Kathmandu]')

outputs

0   2020-01-01
dtype: datetime64[ns]

Expected behavior

0   2020-01-01 05:45:00+05:45
dtype: datetime64[ns, Asia/Kathmandu]

like pandas does

Environment overview (please complete the following information)

• Environment location: [Bare-metal, Docker, Cloud(specify cloud provider)]
• Method of cuDF install: [conda, Docker, or from source]
  • If method of install is [Docker], provide docker pull & docker run commands used

Environment details
Please run and paste the output of the cudf/print_env.sh script here, to gather any other relevant environment details

Additional context
Spotted in Narwhals