[jdk21u-dev] RFR: 6956385: URLConnection.getLastModified() leaks file handles for jar:file and file: URLs

[Daniel Hu]

Clean backport. Fixes file handle leak if `FileURLConnection` used for `JarURLConnection`. Passes tier1 jtreg and the new test/jdk/sun/net/www/protocol/jar/FileURLConnectionLeak.java on linux x64.

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Commit messages:
 - Backport 9f98136c3a00ca24d59ffefd58308603b58110c7

Changes: https://git.openjdk.org/jdk21u-dev/pull/1546/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1546&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6956385
  Stats: 86 lines in 3 files changed: 84 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1546.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1546/head:pull/1546

PR: https://git.openjdk.org/jdk21u-dev/pull/1546