These are Embulk's "standard" plugins which are embedded in Embulk's executable binary distributions.
Their source code had been managed in the same main repository of Embulk until v0.10.33. They have been split from the main repository since v0.10.34.
You need an account in Sonatype OSSRH, and configure it in your ~/.gradle/gradle.properties.
ossrhUsername=(your Sonatype OSSRH username)
ossrhPassword=(your Sonatype OSSRH password)
You need your PGP signatures to release artifacts into Maven Central, and configure Gradle to use your key to sign.
signing.keyId=(the last 8 symbols of your keyId)
signing.password=(the passphrase used to protect your private key)
signing.secretKeyRingFile=(the absolute path to the secret key ring file containing your private key)
Modify version in build.gradle at a detached commit to bump up the versions of Embulk standard plugins.
git checkout --detach master
(Remove "-SNAPSHOT" in "version" in build.gradle.)
git add build.gradle
git commit -m "Release vX.Y.Z"
git tag -a vX.Y.Z
(Write the release note for vX.Y.Z in the tag annotation.)
./gradlew clean && ./gradlew release
git push -u origin vX.Y.Z