Use core 2.28.0

[johnkerl]

This will fail until I get the rwinlib-tiledb PR as in

• Use TileDB Embedded 2.28.0-rc0 #818
• Update to TileDB Embedded 2.28.0-rc0 rwinlib-tiledb#2

Update: done

• Use core 2.28.0 rwinlib-tiledb#3
• https://www.notion.so/Release-process-for-TileDB-R-11332650117580b79eece5ccff0e3e5f

[johnkerl]

I'm confused because TileDB-Inc/rwinlib-tiledb#3 doesn't change anything since the core 2.28.0-rc0 and 2.28.0 tags are tags on the same Git hash, but, CI is failing here 🤔

[johnkerl]

... figured it out, it's the release tag https://github.com/TileDB-Inc/rwinlib-tiledb/releases/tag/v2.28.0 which populates the codeload.github.com URL that CI was failing on until just now.

[mojaveazure]

LGTM; please bump the develop version, update the changelog, and 🚢