Use utf-8 in ServerHttpBasicAuthenticationConverter

[frozenice]

This changes ServerHttpBasicAuthenticationConverter to use utf-8 rather than the platform's default encoding, as per #10903.

• use an explicit charset (utf-8)
• remove useless argument to substring
• add test with non-ASCII characters
• rename test to use proper character name

[sjohnr]

Hi @frozenice. Apologies on the delay reviewing this. I'm working on a webinar that will be done this week and I will circle back next week!

[sjohnr]

Sorry for the delay @frozenice. Thanks for the discussion and the PR. I have a bit of feedback below.

[frozenice]

Fixed the checkstyle errors. Btw the request parameter of org.springframework.security.web.server.authentication.ServerHttpBasicAuthenticationConverterTests#apply is not used.

[sjohnr]

Thanks @frozenice! I've squashed your commits and this is now in main as 2b6bc5d. I have also added a small polish commit 946e24e to add javadoc, copyright year, and an additional test. I'll add this to the 5.7 release as well.