Bump version to 0.7.0b1

[marklysze]

Why are these changes needed?

Version 0.7.0b1 pre-releaese

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.

[qingyun-wu]

Thanks! Can be merged once tests pass!

[marklysze]

PR #374 outstanding, that will need to pass tests and website be accurate for blog post before finalising release.

[marklysze]

The Dependency Injection notebook is showing on the website (though duplicated), will proceed with pre-release.