RFR: JDK-8285730: unify _WIN32_WINNT settings

[Matthias Baesken]

Currently we set _WIN32_WINNT at various places in the codebase; this is used to target a minimum Windows version we want to support. See also for more detailled information :
https://docs.microsoft.com/en-us/windows/win32/winprog/using-the-windows-headers?redirectedfrom=MSDN#setting-winver-or-_win32_winnt
Macros for Conditional Declarations
"Certain functions that depend on a particular version of Windows are declared using conditional code. This enables you to use the compiler to detect whether your application uses functions that are not supported on its target version(s) of Windows."

However currently we have quite a lot of differing settings of _WIN32_WINNT in the codebase ; setting _WIN32_WINNT to 0x0601 (Windows 7) where possible would make sense because we have this setting already at   java_props_md.c  (so targeting older Windows versions at other places is most likely not useful).

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Commit messages:
 - JDK-8285730

Changes: https://git.openjdk.java.net/jdk/pull/8428/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8428&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285730
  Stats: 14 lines in 7 files changed: 1 ins; 0 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8428.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8428/head:pull/8428

PR: https://git.openjdk.java.net/jdk/pull/8428