CI: drop old nim compiler versions

[mratsim]

We remove explicit support for Nim 1.6.x as its EOL and Nim 2.0.x as one version to the next works or does not work.

Furthermore neither 1.6.12 or 2.0.12 can compile #485

Closes #471

[tersec]

Minimal reproducer for this in Nim 2.0 series:

template y(_: static int): untyped = int
type S[F: static int] = y(F)
type E[C: static int] = S[C]
discard sizeof(E[0])

[narimiran]

FWIW, a workaround is to change y(F) to y F or F.y.

@mratsim, would you be interested in making those changes to make constantine compatible with Nim 2.0.x again?

[mratsim]

Yes, but would they still work within a template? Because usually the dot syntax is broken within a template.