[l10n] Add Bulgarian (bg-BG) locale

[GMchris]

Adds bg-BG locale for pickers, referenced in this issue #3211

• I have followed (at least) the PR section of the contributing guide.

[mui-bot]

Localization writing tips ✍️

Seems you are updating localization 🌍 files.

Thank you for contributing to the localization! 🎉 To make your PR perfect, here is a list of elements to check: ✔️

• Verify if the PR title respects the release format. Here are two examples (depending if you update or add a locale file)

  [l10n] Improve Swedish (sv-SE) locale
  [l10n] Add Danish (da-DK) locale

• Update the documentation of supported locales by running pnpm l10n
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• Clean files with pnpm prettier.

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[LukasTy]

Thank you for your contribution! 🙏
Could you please run pnpm prettier and pnpm l10n and commit the changes?
I'm passing the review priviledge to a native speaker. 😉