Use SPDX license descriptors by default

[dbaynard]

At least the default template uses BSD3 for a license identifier. Cabal now supports SPDX license identifiers (and stack is getting features which also support this, e.g. as arguments to ls dependencies). It may be good for the templates to use the SPDX equivalent (BSD-3-Clause) instead.

[dbaynard]

(I'm happy to PR if desired; it's a simple grep)

[BinderDavid]

+1

This also requires to update the cabal-versions in the templates to at least 2.2. Otherwise the following warning will be emitted:

Warning: 'license: BSD-3' is not a recognised license. The known licenses are:                                                        
GPL, GPL-2, GPL-3, LGPL, LGPL-2.1, LGPL-3, AGPL, AGPL-3, BSD2, BSD3, MIT, ISC,                                                        
MPL-2.0, Apache, Apache-2.0, PublicDomain, AllRightsReserved, OtherLicense

An update would remove a little bit of friction that one encounters when trying to update the cabal version to 2.2. in order to use e.g. cabal common stanzas. In that case cabal complains about non SPDX compliant Licence specifiers.

[mpilgrem]

If the BSD3 are replaced by BSD-3-Clause then Stack's in-built Hpack will specify cabal-version: 2.2. I think Cabal-2.2.0.0 was released in March 2018 and included with GHC 8.4.1 (also released in March 2018). Only resolvers before lts-12.1 are pre-GHC 8.4.1. I can't see any harm in bumping to BSD-3-Clause and, consequently, cabal-version: 2.2.

[mpilgrem]

With no objections and the passage of another year, I have put this through. Stack still supports users of GHC < 8.4.1, but the number of such people inconvenienced by this is likely small.