post: Update default branch post-26.05

[mc-nv]

^{CI (internal): #52829374}

What does the PR do?

Advances the main branch after the r26.05 cut: bumps upstream_container_version from 26.04 to 26.05 in build.py, syncs README/docs/deploy configs to reference 26.05, and adds the 26.05 row to the compatibility table. Standard post-release sync of main with the latest release branch.

Changed files include: build.py, README.md, Dockerfile.sdk, deploy/**, docs/introduction/compatibility.md, docs/introduction/release_notes.md, docs/customization_guide/compose.md, docs/user_guide/performance_tuning.md, python/openai/README.md, qa/common/gen_*.

Checklist

• I have made sure this PR is not a duplicate.
• I have made sure the commit message is clear and follows Conventional Commits.
• I have added unit tests for any new functionality.
• I have made sure all tests are passing.
• For documentation changes: I have previewed the changes and they render correctly.

Commit Type:

• feat (A new feature)
• fix (A bug fix)
• docs (Documentation-only changes)
• style (Changes that do not affect the meaning of the code — formatting, whitespace, etc.)
• refactor (A code change that neither fixes a bug nor adds a feature)
• perf (A code change that improves performance)
• test (Adding missing tests or correcting existing tests)
• build (Changes that affect the build system or external dependencies)
• ci (Changes to CI configuration files and scripts)
• chore (Other changes that don't modify src or test files)

Related PRs:

• post: Update default branch post-26.05 model_analyzer#1024
• post: Update default branch post-26.05 pytorch_backend#194
• post: Update default branch post-26.05 tensorrtllm_backend#857

Where should the reviewer start?

build.py, docs/introduction/compatibility.md, README.md

Test plan:

Release-maintenance change; no new behaviour introduced.

• CI Pipeline ID: 52829374

Caveats:

None.

Background

Part of TRI-1063 post-26.04 release maintenance: advance Triton main/master branches to track the r26.05 release and the 26.06 development cycle (upstream_container_version=26.05, triton_container_version=26.06dev).

Related Issues:

Resolves: TRI-1063