PR description:
> This PR attempts to contribute to "Remove gArgs" (#21005).
>
> Main changes:
>
> `GetDataDir()` function is moved to `ArgsManager.GetDataDirPath()`.
> `GetBlocksDir()` function is moved to `ArgsManager.GetBlocksDirPath()`.
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/21244 | core#21244]] [1/8]
https://github.com/bitcoin/bitcoin/pull/21244/commits/70cdf679f8e665dbdc3301873a0267fe9faa72cd