I used to buy CSS Image Gallery 2, but will it work in dreamweaver 2021 for dynamic content?

No, that's an old extension which for dynamic data relies on the old, deprecated server behaviors, which are not available in modern DW versions.

too bad it is not stated that it is no longer supported in the current versions of dw
What is the replacement that works in the current versions?
Is there no "upgrade" for older extensions to the new?

The extension should be working fine in CC2021, the issue is that the server behaviors are NOT available any longer (long time ago actually).

when you choose for dynamic, you can't select a data source

Well of course you can't as server behaviors it relies on are NOT available. As i already explained:

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for dynamic data relies on the old, deprecated server behaviors, which are not available in modern DW versions.

is there a new version of it?

No there isn't a newer version of this extension.