Merge `release/2.21.0` into `2.x`
# Conflicts:
# .github/generate-email.sh
# .github/workflows/build.yml
# README.md
# pom.xml
# src/changelog/.2.x.x/1783_spring_arbiter_without_environment.xml
# src/changelog/.2.x.x/UpdateSurefireRelease.xml
# src/changelog/2.21.0/PreventResourceLeak.xml
# src/site/_release-notes.adoc
# src/site/_release-notes/_2.0-alpha1.adoc
# src/site/_release-notes/_2.0-alpha2.adoc
# src/site/_release-notes/_2.0-beta1.adoc
# src/site/_release-notes/_2.0-beta2.adoc
# src/site/_release-notes/_2.0-beta3.adoc
# src/site/_release-notes/_2.0-beta4.adoc
# src/site/_release-notes/_2.0-beta5.adoc
# src/site/_release-notes/_2.0-beta6.adoc
# src/site/_release-notes/_2.0-beta7.adoc
# src/site/_release-notes/_2.0-beta8.adoc
# src/site/_release-notes/_2.0-beta9.adoc
# src/site/_release-notes/_2.0-rc1.adoc
# src/site/_release-notes/_2.0-rc2.adoc
# src/site/_release-notes/_2.0.1.adoc
# src/site/_release-notes/_2.0.2.adoc
# src/site/_release-notes/_2.0.adoc
# src/site/_release-notes/_2.1.adoc
# src/site/_release-notes/_2.10.0.adoc
# src/site/_release-notes/_2.11.0.adoc
# src/site/_release-notes/_2.11.1.adoc
# src/site/_release-notes/_2.11.2.adoc
# src/site/_release-notes/_2.12.0.adoc
# src/site/_release-notes/_2.12.1.adoc
# src/site/_release-notes/_2.13.0.adoc
# src/site/_release-notes/_2.13.1.adoc
# src/site/_release-notes/_2.13.2.adoc
# src/site/_release-notes/_2.13.3.adoc
# src/site/_release-notes/_2.14.0.adoc
# src/site/_release-notes/_2.14.1.adoc
# src/site/_release-notes/_2.15.0.adoc
# src/site/_release-notes/_2.16.0.adoc
# src/site/_release-notes/_2.17.0.adoc
# src/site/_release-notes/_2.17.1.adoc
# src/site/_release-notes/_2.17.2.adoc
# src/site/_release-notes/_2.18.0.adoc
# src/site/_release-notes/_2.19.0.adoc
# src/site/_release-notes/_2.2.adoc
# src/site/_release-notes/_2.20.0.adoc
# src/site/_release-notes/_2.3.adoc
# src/site/_release-notes/_2.4.1.adoc
# src/site/_release-notes/_2.4.adoc
# src/site/_release-notes/_2.5.adoc
# src/site/_release-notes/_2.6.1.adoc
# src/site/_release-notes/_2.6.2.adoc
# src/site/_release-notes/_2.6.adoc
# src/site/_release-notes/_2.7.adoc
# src/site/_release-notes/_2.8.1.adoc
# src/site/_release-notes/_2.8.2.adoc
# src/site/_release-notes/_2.8.adoc
# src/site/_release-notes/_2.9.0.adoc
# src/site/_release-notes/_2.9.1.adoc
# src/site/_release-notes/_2.x.x.adoc
