fix(lua): drop support for boolean buf in vim.keymap #38432

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.

runtime/lua/vim/_meta/api.lua

@@ -2018,7 +2018,7 @@ function vim.api.nvim_parse_expression(expr, flags, highlight) end
 ---     line2
 ---     line3
 ---   ]], false, -1)
---- end, { buf = true })
+--- end, { buf = 0 })
 --- ```
 ---
 --- @param data string Multiline input. Lines break at LF ("\n"). May be binary (containing NUL bytes).