Change bootstrap to 1.7.21-release-254

This is supposed to be the same build as the one that was accidentally
uploaded with version 1.7.21 to the Space bootstrap repository (and
which predates the real 1.7.21).

gradle.properties

@@ -19,7 +19,7 @@ kotlin.build.gradlePlugin.version=0.0.38
 #maven.repository.mirror=http://repository.jetbrains.com/remote-repos/
 #bootstrap.kotlin.repo=https://maven.pkg.jetbrains.space/kotlin/p/kotlin/bootstrap
 #bootstrap.kotlin.version=1.1.50-dev-1451
-bootstrap.kotlin.default.version=1.7.21
+bootstrap.kotlin.default.version=1.7.21-release-254
 
 kotlin.build.publishing.attempts=20
 #signingRequired=true