Rename json object `reasons` to `reason` to comply with GraalVM's schema

[zakkak]

Description

#42205 introduced a new json entry to the Quarkus-generated config files passed to GraalVM native-image.

The said entry is called reasons while GraalVM's reachability-metadata.json uses reason.

Reported by @linghengqian

Implementation ideas

No response

[quarkus-bot]

/cc @Karm (native-image), @galderz (native-image)

[geoand]

@zakkak do you have a fix for this in the works?

[zakkak]

No, I was thinking of fixing this in the implementation of #41016 once we move to Mandrel for JDK 25.

[geoand]

👍🏽