Dependabot - Private "Registries"/"Dependencies"/"Repositories" is misleading

### Code of Conduct

- [x] I have read and agree to the GitHub Docs project's [Code of Conduct](https://github.com/github/docs/blob/main/.github/CODE_OF_CONDUCT.md)

### What article on docs.github.com is affected?

https://docs.github.com/en/code-security/reference/supply-chain-security/supported-ecosystems-and-repositories

### What part(s) of the article would you like to see updated?

Dependabot sometimes cannot resolve private dependencies for some ecosystems (e.g. Nix, currently)

However, this is expressed in 3 different expressions:
1. Private **Registries**
2. Private **Dependencies**
3. Private **Repositories**

Those can be misleading

For example:
I personally thought dependabot was completely not working for some ecosystems independently of private dependencies when the `dependabot.yml` file was inside of a private repository

Harmonizing the "Private *" term with a single word (probably "Private Dependencies", since it encompasses both registries and repositories) would have cleared up any confusion

### Additional information

_No response_

閱讀 docs.github.com/en/code-security/reference/supply-chain-security/supported-ecosystems-and-repositories 中受影響的文章，著重查看「Private Registries」、「Private Dependencies」和「Private Repositories」這三處用法。統一術語，明確私有相依項目和私有儲存庫的範圍，然後檢查轉譯後的文章，確保措辭一致。

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