RFR: 8345883: Relax system property "stackwalk.debug" in StackStreamFactory to be case insensitive

[Eirik Bjørsnøs]

Please review this PR which relaxes the interpretation of the system property `stackwalk.debug` in `java.lang.StackStreamFactory` to be case insensitive.

Motivation:
Only 5 of 83 boolean system properties in `java.base` have a case sensitive interpretation. Relaxing these to be case insensitive will improve consistency across the code base and simplifies our reasoning about such system properties.

The `stackwalk.debug` property was introduced when JEP-259 was integrated in JDK-8143911. The property was not mentioned in the CCC for this change, neither in the JEP. Seems like an undocumented, internal debug facility for development and maintenance purposes.

Risk:
This should be a low risk change. The property is used for debugging, and the change will shift the interpretation of "TRUE" from false to true. Any user specifying `-Dstackwalk.debug=TRUE` probably meant to enable it.

Verification:
Manually verified that `-Dstackwalk.debug=TRUE` and `-Dstackwalk.debug=true` enables debugging and that `-Dstackwalk.debug=false`, `-Dstackwalk.debug` and `-Dstackwalk.debug=abc` does not enable it.

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Commit messages:
 - Boolean system property "stackwalk.debug" should ignore case when evaluating the value to true or false

Changes: https://git.openjdk.org/jdk/pull/22659/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22659&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345883
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22659.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22659/head:pull/22659

PR: https://git.openjdk.org/jdk/pull/22659