[maven-release-plugin] prepare release Singularity-0.21.0

[maven-release-plugin] prepare release Singularity-0.20.1

[maven-release-plugin] prepare release Singularity-0.20.0

[maven-release-plugin] prepare release Singularity-0.19.2

[maven-release-plugin] prepare release Singularity-0.19.1

[maven-release-plugin] prepare release Singularity-0.17.1

[maven-release-plugin] prepare release Singularity-0.19.0

[maven-release-plugin] prepare release Singularity-0.18.2

[maven-release-plugin] prepare release Singularity-0.18.1

[maven-release-plugin] prepare release Singularity-0.18.0