[rfc] change copyright text to gplv2 or v3

[Martin Pool]

> > What if someone other than Canonical wants to link Bazaar with
> > (hypothetical) GPLv4?

Basically the same as if they want to link it with another open source
license.  We can consider both the particular case and the merits of
that license in general.  I would expect that conversation would be
carried out through the community of Bazaar contributors just like
this.

-- 
Martin