Drop support for ESLint<8

[ota-meshi]

close #38

[voxpelli]

Any reason to keep supporting anything older than ESLint 8?

[ota-meshi]

Thank you for your comment.

I don't know if it makes sense for you, but I often refer to typescript-eslint to decide which version of eslint to support.
https://github.com/typescript-eslint/typescript-eslint/blob/503bfde75c66d9fb6a36dc1a94de6e47bdb5376d/packages/eslint-plugin/package.json#L74
Because I often use typescript-eslint related modules in test cases.

If we support eslint v6 or later, we don't need the if: branch on CI.

[voxpelli]

I would opt for bumping to the newest ESLint version, then the need to bump it again and do a new major will be very low for that reason for a considerable amount of time, and ESLint 8 has been out long enough that there should be little point in supporting older versions of ESLint?

[ota-meshi]

I have updated this PR to change the minimum version of eslint that the plugin supports to v8.