fix: use explicit UTF-8 encoding in AuthParser URL decode/encode

[ChihweiLHBird]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.

Description of the PR

• Replace deprecated URLDecoder.decode(String) with URLDecoder.decode(String, String) using StandardCharsets.UTF_8 in AuthParser.parse()
• Update URLEncoder.encode() in AuthParser.reconstruct() to use StandardCharsets.UTF_8.name() instead of a hardcoded "UTF-8" string literal
• Remove 10-year-old FIXME comment (added in PR fix java warnings #1900, Jan 2016)

[Copilot]

Pull request overview

This PR updates AuthParser to use explicit UTF-8 encoding for URL decode/encode operations, eliminating reliance on platform defaults and removing an outdated FIXME.

Changes:

• Replace deprecated URLDecoder.decode(String) usage with URLDecoder.decode(String, String) using UTF-8.
• Update URLEncoder.encode(..., "UTF-8") calls to use StandardCharsets.UTF_8.name().
• Remove the old FIXME comment about deprecated decoding.

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