Previously, the targetJavaVersion property of bootBuildImage was set
using the project's target compatibility directly. This meant that
property's value would reflect the configured target compatibility at
the time of the task's creation and any subsequent changes to the
target compatibility would be missed.

This commit updates the registration of the bootBuildImage task to set
the value of the targetJavaVersion property using a provider. This
indirection means that the project's target compatibility isn't read
until the value of the targetJavaVersion is read, allowing any changes
to the target compatibility to be picked up even if they're made after
the bootBuildImage task has been created.

Fixes gh-26297