Add crossSbtVersions to excludeLintKeys (#477)
Add crossSbtVersions to excludeLintKeys (#477)

This prevents sbt from complaining about the `crossSbtVersions` key
not being used by any other settings/tasks:

```
[warn] there's a key that's not used by any other settings/tasks:
[warn]
[warn] * xsbt-web-plugin / crossSbtVersions
[warn]   +- build.sbt:2
[warn]
[warn] note: a setting might still be used by a command; to exclude a key from this `lintUnused` check
[warn] either append it to `Global / excludeLintKeys` or call .withRank(KeyRanks.Invisible) on the key
```
earldouglas/xsbt-web-plugin,earldouglas/xsbt-web-plugin