Consistently replace String encoding names with StandardCharset arguments

[jhoeller]

On a related note to gh-27646, our EncodedResource should consistently be used with StandardCharsets arguments instead of String encoding names. Also, there are URLEncoder/URLDecoder variants with a Charset argument in JDK 10+ now.

[dreis2211]

Please check #27554

[jhoeller]

Oops, sorry for not noticing your PR before, @dreis2211 ... This consistency commit of mine only really covered a single URLEncoder case but the PR #27646 included some URLDecoder changes as well, next to the toString stuff.

If you could rebase #27554 against current main, we'll see how much we're still missing. From a quick glance, there are quite a few hard-coded "UTF-8" constants in use against URLDecoder still. It'd be great to apply your PR for that purpose!

[dreis2211]

@jhoeller Done

[jhoeller]

@dreis2211 Wow that was quick, thanks for the immediate turnaround!