[UX] Make retry clean up security group silent

[cblmemo]

There will be some output that interrupts the progress bar when clean-up SG fails:

» sky down sky-serve-controller-8a3968f2-0
Tearing down sky serve controller: sky-serve-controller-8a3968f2-0.
To proceed, please check the information above and type 'delete': delete
Terminating 1 cluster ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━   0% -:--:--I 10-19 21:52:21 instance.py:697] Security group sky-sg-sky-serve-controller-8a3968f2-0-8a39 is still in use. Retry.
Terminating cluster sky-serve-controller-8a3968f2-0...done.
Terminating 1 cluster ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100% 0:00:00

This PR makes the log silent by changing it to debug. See #2458 (review) for more information.

Tested (run the relevant ones):

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

[concretevitamin]

Thanks @cblmemo.