[CBM] [FDB] Deprecate ForestDB as a supported archive format

What's the issue?
ForestDB was the default storage format in 'cbbackupmgr' prior to 6.5.0 at which point it was switch to SQLite.

Maintaining support for reading ForestDB archives is costly:

1. Multiple large changes were enacted at the same time e.g. the removal of sharding and the splitting out of metadata
2. It's an additional CGO dependency
3. It's a library that could potentially be removed to reduce build times/maintenance overhead
4. It has to be specifically handled just to allow building for CV (and has been broken a couple of times due to unrelated CMake changes)

What's the fix?
We deprecate support for reading archives created prior to 6.0.0, and remove ForestDB from 'cbbackupmgr' in the next major release.

Who will this affect?
This will affect customers who have old backups "squirreled" away that were created using a version of 'cbbackupmgr' prior to 6.5.0.

What's the "workaround"?
There's two potential workarounds:

1. Use a 7.1.x or earlier version of 'cbbackupmgr' to perform the restore
2. Use a 7.1.x or earlier version to perform a 'merge', then restore with the latest version

To expand on the second case, 'cbbackupmgr' only supports writing one version (the current version) but supports reading multiple versions; this means when you perform a 'merge' you're implicitly upgrading the backups being merged. This behavior can be leveraged to "upgrade" an archive so that it can be used with the latest version of 'cbbackupmgr'.