You will need to open the file /var/lib/dpkg/status and do a search for elasticsearch.
This should bring you to a section that looks like this:
Package: elasticsearch
Status: install ok half-configured
Priority: optional
Section: web
Installed-Size: 196305
Maintainer: Elasticsearch Team <info@elastic.co>
Architecture: amd64
Source: elasticsearch
Version: 7.2.0
Depends: bash (>= 4.1), lsb-base (>= 4), adduser, coreutils (>= 8.4)
What you want to do here is remove the dependency from the "Depends" line (last line). In this example, I already removed it but you should see libc6 or c6. Just remove only that one and keep the others there. Then save the file.
When you use apt-get, it may complain about the previous install, but it won't force you to remove it. This isn't the most glamorous solution right now but it won't break anything. There's most likely a simpler method but this works great for the time being.
The other option is this: https://raspberrypi.stackexchange.com/questions/12573/which-is-the-highest-version-of-libc6-available-for-raspbian
You can add an unstable repo to install libc6. I don't know what the effects are here but it shouldn't break anything. If you go that route, you may have to do a reinstall of elasticsearch. Just back up your /var/lib/elasticsearch/data directory and config files and reinstall (I don't think a reinstall will touch the config files unless you did a purge).