Remove md5 password encryption support

[emteknetnz]

Follow on from #11524 (comment)

Acceptance criteria

• Using md5 for password encryption shows a deprecation message in the old major release line
• md5 support is removed in the new major release line
• Done for both the md5 and md5_v2.4 keys in encryptors.yml
• A new 'best practice' algorithm is chosen, if it hasn't already been chosen e.g. sha1, blowfish, something else
• Consideration is given to the developer experience of migrating from a database full of md5 encrypted passwords
• Any required migration documentation is added to the changelog