Update instructions and behavior for Fiji-Latest and Java 21 and cjdk

[ctrueden]

• scyjava now supports downloading openjdk on demand via cjdk. PyImageJ could do set_java_constraints(version='21') as a default behavior.
• Then we could remove the openjdk conda package installation req from the docs.
• We need to test the JPMS difficulties from PyImageJ—which --add-opens + --add-exports do we actually need, given that app-launcher tries to reflectively unlock everything (except java.base/java.lang)?
• The imagej.doctor would benefit from an update that understands the new landscape of Fiji / OpenJDK 21 / Jaunch / cjdk.