[azure] support latest version for image id in azure cloud

[Yisaer]

As titled.

close #4435

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)

I manually tested with following task:

resources:
  cloud: azure
  region: japaneast
  instance_type: Standard_D2s_v3
  image_id: Canonical:ubuntu-24_04-lts:server:latest

# Working directory (optional) containing the project codebase.
# Its contents are synced to ~/sky_workdir/ on the cluster.
workdir: .

# Typical use: pip install -r requirements.txt
# Invoked under the workdir (i.e., can use its files).
setup: |
  echo "Running setup."

# Typical use: make use of resources, such as running training.
# Invoked under the workdir (i.e., can use its files).
run: |
  echo "Hello, SkyPilot!"
  conda env list

The cluster was launched and the job work well. I checked the image in the azure console, and it was ubuntu-24_04-lts as expected

• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: conda deactivate; bash -i tests/backward_compatibility_tests.sh