Merge bitcoindevkit/bdk#711: Release/0.21.0

8026bd9476fc5a89baa86ea2a8707a9d341d3743 Bump version to 0.21.1-dev (Alekos Filini)
e2bd96012a08137e4f6776c9192d5247ae20df0b Bump version to 0.21.0 (Alekos Filini)
2c01b6118f3291c2918a60cac635f26df1780dac Bump version to 0.21.0-rc.1 (Alekos Filini)

Pull request description:

  ### Description

  Merge the release branch back into master

  ### 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:

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

  #### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [ ] I've added tests to reproduce the issue which are now passing
  * [ ] I'm linking the issue being fixed by this PR

ACKs for top commit:
  danielabrozzoni:
    ACK 8026bd9476fc5a89baa86ea2a8707a9d341d3743

Tree-SHA512: a2a924a60d551a823de035b609d4d51652a165a0695212af76dea87706919c8929dba977bb297f4787708470bf075d14dd0a37657bd3a76e7d44a746fb5439df