fixed deprecated version of vim.validate()

[pebbe]

Description

The use of vim.validate(spec) is deprecated.

Type of change

Changed all calls to vim.validate(spec) into calls to
vim.validate(name, value, validator[, optional][, message])

Configuration:

• Neovim version (nvim --version): v0.11.2
• Operating system and version: Debian GNU/Linux bookworm 12.11 x86_64

Checklist:

• My code follows the style guidelines of this project (stylua)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (lua annotations)

[clason]

Needs to be rebased and reopened (no CI runs == no merge).