Update doc for KeePassXC

Mostly KeePassX replacement with KeePassXC is a dropin replacement. The usage is the same as before, the filename is the same.
So replace
KeePassX -> KeePassXC
_{/.config/keepassx/keepassx2.ini ->}/.config/keepassxc/keepassxc.ini

Migration from old file format is still supported via Import.

Don’t know if it is too technical:

We may also want to document that there is a new kdbx file format, that is incompatible with the current one.
KeePassXC won’t upgrade the file format automatically.

You need to update to this format by hand by switching Encryption Algorithm to “ChaCha20”, and the Key Derivation Function to “Argon2”:

https://keepassxc.org/docs/#faq-security-kdbx4
https://theorangeone.net/posts/keepassxc-2.3-migration/#kdbx4

Parent Task: #16282 (closed)

Related issues

• Blocked by #15297 (closed)

Original created by @Hefee on 16284 (Redmine)