v0.18.21

DataStructures v0.18.21

Diff since v0.18.20

Merged pull requests:

• CI: remove invalidations test (#927) (@fingolfin)
• CI: run doctests to catch regressions; test against Julia LTS; allow julia-actions/cache to cleanup caches (#931) (@fingolfin)

Closed issues:

• Continuous integration should break when doctests are broken (#345)
• Changelog for releases 0.18.16 and following (#912)