Bump version to 0.8.4

[marklysze]

Why are these changes needed?

Bump version to 0.8.4

Related issue number

N/A

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/docs/contributor-guide/documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
autogen/version.py	100.00% <100.00%> (ø)

... and 97 files with indirect coverage changes

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