OpenJDK 17 is in Debian 11 as well [0]. A decision was made to include it before its official release since the release is so close (September). An updated package will be available when it's released.
From the release notes [1]:
Debian bullseye comes with an early access version of OpenJDK 17 (the next expected OpenJDK LTS version after OpenJDK 11), to avoid the rather tedious bootstrap process. The plan is for OpenJDK 17 to receive an update in bullseye to the final upstream release announced for October 2021, followed by security updates on a best effort basis, but users should not expect to see updates for every quarterly upstream security update.
From the release notes [1]:
Debian bullseye comes with an early access version of OpenJDK 17 (the next expected OpenJDK LTS version after OpenJDK 11), to avoid the rather tedious bootstrap process. The plan is for OpenJDK 17 to receive an update in bullseye to the final upstream release announced for October 2021, followed by security updates on a best effort basis, but users should not expect to see updates for every quarterly upstream security update.
[0] https://packages.debian.org/source/testing/openjdk-17
[1] https://www.debian.org/releases/bullseye/amd64/release-notes...