RFR: 8361424: Eliminate Log methods mandatoryWarning() and mandatoryNote()

[Archie Cobbs]

The fix for [JDK-8359493](https://bugs.openjdk.org/browse/JDK-8359493) added the ability to associate `DiagnosticFlag`'s to specific warnings at the point of definition in `compiler.properties`. In cases where the flag is an inherent property of the warning itself, `compiler.properties` is a more appropriate place to declare it, instead of having to add it at all the points in the codebase where that particular warning is used.

In particular, the Log API methods `mandatoryWarning()` and `mandatoryNote()` can be removed, as they serve only to add the `MANDATORY` flag to warnings and notes for which "mandatoryness" is inherent. Doing so should help decomplexify the code a bit.

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Commit messages:
 - Merge branch 'master' into JDK-8361424
 - Eliminate Log.mandatoryNote() and add MANDATORY flags to compiler.properties.
 - Eliminate Log.mandatoryWarning() and add MANDATORY flags to compiler.properties.
 - Remove Log::strictWarning
 - Merge with latest compiler changes
 - Initial push

Changes: https://git.openjdk.org/jdk/pull/26138/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26138&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361424
  Stats: 103 lines in 9 files changed: 8 ins; 48 del; 47 mod
  Patch: https://git.openjdk.org/jdk/pull/26138.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26138/head:pull/26138

PR: https://git.openjdk.org/jdk/pull/26138