[electrum] bump mnemonic dependency to >=0.21

Summary:
Mnemonic is the python lib we use for BIP39 mnemonic seed phrase operations. It is maintained by trezor.

We already pinpointed mnemonic==0.21 in requirements-hw.txt, so make this requirements consistent. Keeping sensitive libraries up to date is also good for security.

The main release note items for mnemonic v0.21 are:

• Use secrets module for randomness instead of os.urandom (see https://github.com/trezor/python-mnemonic/pull/89)
• Language detection is unambiguous even if some words are ambiguous
• Support for Python below 3.8 was dropped

Depends on D16669

Test Plan: Run the release scripts, test the files

Reviewers: #bitcoin_abc, roqqit

Reviewed By: roqqit

Differential Revision: https://reviews.bitcoinabc.org/D16664