I'd recommend upgrading the NWT on your hosts, but you're not regulated to ensure it's kept in lockstep with the NimbleOS array release. The Nimble Windows Toolkit is supported and tested for within two major releases of code - so in your case the 2.1.x NWT would be fully supported to use in conjunction with 2.2.x or 2.3.x NimbleOS. Although no new host features would be available to you if you were to continue using the old NWT.
I notice that you're running Windows 2008 - if this is the non-R2 release, then NWT 2.1.x is the final release that supports that operating system. from 2.2.x onwards it's Windows 2008 R2, Windows 2012 and Windows 2012 R2 support.
Hope this helps!