Use khr_default_context() per DPC++ 2025.2 compiler deprecation warning

[antonwolfy]

This PR follows a deprecation warning raised by DPC++ 2025.2 compiler and updates DPCTLPlatform_GetDefaultContext implementation to replace deprecated API call of ext_oneapi_get_default_context() (see llvm#17135) with khr_get_default_context() one (added in llvm#15645).

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[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞

[github-actions]

Array API standard conformance tests for dpctl=0.20.0dev0=py310h93fe807_108 ran successfully.
Passed: 894
Failed: 2
Skipped: 154

[coveralls]

coverage: 86.381% (+0.003%) from 86.378%
when pulling f8c20ce on resolve-cmplr-deprecation-warning
into e39b013 on master.

[github-actions]

Array API standard conformance tests for dpctl=0.20.0dev0=py310h93fe807_110 ran successfully.
Passed: 894
Failed: 2
Skipped: 154

[ndgrigorian]

Deprecation warnings are gone, LGTM