Merge bitcoin/bitcoin#21850: Remove `GetDataDir(net_specific)` function

aca0e5dcdb174ef7e88b76910d6fffd633688235 Remove `GetDataDir(bool fNetSpecific = true)` function (Kiminuo)
b3e67f20a02bcf43f20873029ce30617c95eb9da scripted-diff: Replace `GetDataDir(true)` calls with `gArgs.GetDataDirNet()` calls (Kiminuo)
4c3a5dcbfc3010965332ad568c3a70618c930ef3 scripted-diff: Replace `GetDataDir()` calls with `gArgs.GetDataDirNet()` calls (Kiminuo)
13bd8bb0536f008118b1de921654925ed9ce1da7 Make `ArgsManager.GetDataDirPath` private and drop needless suffix (Kiminuo)
4d8189f62076d47be182e709857774bc3921b334 scripted-diff: Change `ArgsManager.GetDataDirPath()` to `ArgsManager.GetDataDirBase()` in tests (Kiminuo)
0f53df47d5b8319fc71f3b87c15a115ff797fb50 Add `ArgsManager.GetDataDirBase()` and `ArgsManager.GetDataDirNet()` as an intended replacement for `ArgsManager.GetDataDirPath(net_identifier)` (Kiminuo)
716de29dd8672d28bfe0a08424e9958fe61054d2 Make `m_cached_blocks_path` mutable. Make `ArgsManager::GetBlocksDirPath()` const. (Kiminuo)

Pull request description:

  This PR is a follow up PR to #21244. The PR attempts to move us an inch towards the [goal](https://github.com/bitcoin/bitcoin/pull/21244#discussion_r615307465) by removing `GetDataDir(net_specific)` and replacing it by `gArgs.GetDataDir(net_specific)` calls.

  The approach of this PR attempts to be similar to the one chosen in "De-globalize ChainstateManager" (#20158). The goal is to pass `ArgsManager` to functions (or ideally to have `ArgsManager` as a member of a class where needed; inspiration from here: #21789) instead of having it as a global variable (i.e. `gArgs`).

  **Notes:**
  * First commit makes `m_cached_blocks_path` `mutable` as was suggested [here](https://github.com/bitcoin/bitcoin/pull/21244#discussion_r615274095) but not fully applied in #21244. (`m_cached_datadir_path` and `m_cached_network_datadir_path` were marked as `mutable` in #21244) This commit can be in a separate PR too.
  * Other commits deal with removing of `GetDataDir(net_specific)` function.
      * This was originally part of #21244 but it was [left]((https://github.com/bitcoin/bitcoin/pull/21244#pullrequestreview-633779754)) for a follow up PR.
  * I think that the proposed changes show nicely where there is reliance on `gArgs` which is IMO a good thing.

  If you know about a better approach how to do this, please share it here.

ACKs for top commit:
  hebasto:
    ACK aca0e5dcdb174ef7e88b76910d6fffd633688235
  MarcoFalke:
    re-ACK aca0e5dcdb174ef7e88b76910d6fffd633688235 👃

Tree-SHA512: deec4d88edb32d7f4c818c3a74ffbb64709685819b88242dcf5dbaa1fb611f3ce2b29d2576ddb9e0dc5e75288e43538968224008c0a80e7149fc81c309f7c9da

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