Merge bitcoindevkit/bdk#628: rpc: use `importdescriptors` with Core >= 0.21

Merge bitcoindevkit/bdk#628: rpc: use `importdescriptors` with Core >= 0.21

e1a1372baec86da784af697e025615b8e0bccf4e rpc: use `importdescriptors` with Core >= 0.21 (Alekos Filini)

Pull request description:

  ### Description

  Only use the old `importmulti` with Core versions that don't support descriptor-based (sqlite) wallets.

  Add an extra feature to test against Core 0.20 called `test-rpc-legacy`.

  This also makes us compatible with Core 23.0 and is thus a replacement for #613, which actually looking back at it was adding support for 23.0 but probably breaking older wallets by adding the extra argument to `createwallet`.

  I believe #613 should now only focus on getting the tests to work against 23.0, which is still important but not such a high priority as being compatible with the latest version of Core.

  Also fixes #598

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] I've added docs for the new feature
  * [x] I've updated `CHANGELOG.md`

ACKs for top commit:
  notmandatory:
    ACK e1a1372baec86da784af697e025615b8e0bccf4e

Tree-SHA512: 7891b8ab2fc900ea2a186f64a32aea970f28a50339063ed0e1a8d13248e5c038b8fff3d9e26b93cb7daafd0c873379e64a28836dbe4e4b82f1983577a88971ff