RFR: 8358688: HttpClient: Simplify file streaming in RequestPublishers.FilePublisher [v6]

[Volkan Yazici]

> Simplifies content streaming for `HttpRequest.BodyPublishers::ofFile(Path)`, which delegates to `RequestPublishers.FilePublisher::create(Path)`. This effectively replaces all usages of `FileInputStream::new` in `HttpClient` with `Files::newInputStream(Path)`. See the JBS ticket description for the full story.
> 
> Also removing certain test files which were rendered redundant after `SecurityManager` removal. We do this in this PR, since tests were added due to touched lines.
> 
> `tier1,2` results on b5f3c8dc6f4 are attached to the ticket.

Volkan Yazici has updated the pull request incrementally with one additional commit since the last revision:

  Simplify `FilePublisherTest`

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Changes:
  - all: https://git.openjdk.org/jdk/pull/25662/files
  - new: https://git.openjdk.org/jdk/pull/25662/files/de20dae0..1eadf2dc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25662&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25662&range=04-05

  Stats: 9 lines in 1 file changed: 0 ins; 5 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/25662.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25662/head:pull/25662

PR: https://git.openjdk.org/jdk/pull/25662