RFR: 8345249: Apply some conservative cleanups in FileURLConnection

[Eirik Bjørsnøs]

Please review this PR which applies various cleanups to `sun.net.www.protocol.file.FileURLConnection`.

This class is known to be an _old, intricate, and hard to maintain piece of code._  However, there are some relatively straightforward refactorings / cleanups possible which improve readability and makes it easier to reason about what's going on in this class.

In this PR, I have chosen to make each individual small change a separate commit. This to assist review of each individual change, which can otherwise disappear a bit when reviewing the PR as a whole. 

A detailed listing of each commit follows in a separate comment.

This is a pure cleanup PR, no tests are added or updated in this PR.

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Commit messages:
 - Use enhanced for loop when constructing directory list
 - Avoid explicit type parameter when calling Arrays::asList
 - Update copyright year
 - Remove unused local variables in getInputStream
 - Make instance fields private
 - Reorder instance fields
 - Remove unneccessary catch/rethrow of IOException
 - Remove contentType field
 - Rename files field to directoryListing
 - Make file field final
 - ... and 1 more: https://git.openjdk.org/jdk/compare/4bc826ac...f9efa3b8

Changes: https://git.openjdk.org/jdk/pull/22459/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22459&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345249
  Stats: 38 lines in 1 file changed: 2 ins; 13 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/22459.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22459/head:pull/22459

PR: https://git.openjdk.org/jdk/pull/22459