Remove support for OPAM 1.x files

[Leonidas-from-XIV]

We currently support the OPAM 1.2 and 1.0 format in dune-release. Given these formats are long-obsolete and OPAM Repository doesn't maintain version 1.x packages there is little use in continuing to support these:

1. Pull requests to OPAM repository have to be OPAM 2.0 format
2. dune generates 2.0 format by default
3. By and large the community has updated to OPAM 2.0 format

Removing the support would make our code base a bit easier to maintain and remove code paths that don't receive much testing. Affected users if any can update their OPAM files to 2.0 format which will net them a lot of benefits besides just what dune-release offers them.

[NathanReb]

Same here as with #406, we should still check the version field but simply error out when it's too old instead of running the conversion to 2.x.