http_server: compatibility fixes for vllm>0.6.1.post1

[dtrifiro]

• [BugFix] Fix clean shutdown issues vllm-project/vllm#8492 renamed init_app to init_app_state
• gha: bump vllm tag to v0.6.1.post2

https://issues.redhat.com/browse/RHOAIENG-12997

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 58.33333% with 5 lines in your changes missing coverage. Please review.

Project coverage is 56.66%. Comparing base (355a088) to head (21b0fce).

Files with missing lines	Patch %	Lines
src/vllm_tgis_adapter/http.py	54.54%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #136      +/-   ##
==========================================
- Coverage   56.80%   56.66%   -0.14%     
==========================================
  Files          25       25              
  Lines        1528     1537       +9     
  Branches      277      256      -21     
==========================================
+ Hits          868      871       +3     
- Misses        582      588       +6     
  Partials       78       78              

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[dtrifiro]

main branch failing because of vllm-project/vllm#8491. Waiting on vllm-project/vllm#8537 to fix the issue

[njhill]

@dtrifiro the change in question here was made post 0.6.1-post2 i.e. it's not in a release yet. We should be able to move to 0.6.1-post2 without this change.