Problem:
`vim.keymap.*.Opts.buf` allows `boolean` aliases for more widely
used `integer?` values, `true` -> `0` and `false` -> `nil`. This
conversion is unnecessary and can be handled at call sites.
Solution:
As a follow-up to deprecating the `buffer` option, drop support for
boolean values for the new `buf` option. The deprecated `buffer`
continues to support booleans for backward compatibility.
9a5641b4b5