Drop support for Go 1.7 and Go 1.8

In the next patch I will need to use big.Int.IsInt64, which was added in
Go 1.9 in 2017. Go 1.7 and Go 1.8 are very old nowdays - their last
point releases were done in 2017 (go1.7.6) and 2018 (go1.8.7) - 7 and 6
years ago correspondingly. Since then those releases are EOL with many
further Go releases done after them. We even specify go1.12 as the
requirement in our go.mod, but that is probably because go1.12 is the
first version to support modules. Anyway dropping support for those very
old Go1.7 and Go1.8 should be almost certainly ok as I doubt if there
are current ogórek users still stuck at those old Go versions.

.github/workflows/ci.yml

@@ -11,8 +11,6 @@ jobs:
     strategy:
       matrix:
         go:
-          - "1.7.x"
-          - "1.8.x"
           - "1.9.x"
           - "1.11.x"
           - "1.12.x"