Bump version to 1.6.21 [#159875907]

Co-authored-by: Leah Hanson <lhanson@pivotal.io>

Bump version to 1.6.20 [#159222872]

Bump version to 1.6.19 [#158751711]

Bump version to 1.6.18 [#158582334]

Bump version to 1.6.17 [#157902676]

Signed-off-by: Leah Hanson <lhanson@pivotal.io>

Bump version to 1.6.16 [#157542381]

Signed-off-by: Andrew Meyer <ameyer@pivotal.io>

Bump version to 1.6.15 [#157369079]

Bump version to 1.6.14 [#156865415]

Bump version to 1.6.13 [#156788862]

Bump version to 1.6.12 [#156076381]