Restore compatibility with Mandrel 20.3

[gsmet]

Fix #18513

@zakkak I let you push this PR to your CI? Thanks!

[zakkak]

Closes #18513

@zakkak I let you push this PR to your CI? Thanks!

Started CI run: https://github.com/graalvm/mandrel/actions/runs/1011083032

Thanks

[zakkak]

LGTM. Thanks Guillaume!

Could we also add some comments tight above the changes that are for 20.3 compatibility so that we can easily grep them (and ultimately remove them) in the future?

Waiting for the CI to finish.... https://github.com/graalvm/mandrel/actions/runs/1011083032

[gsmet]

@zakkak I think we'd better just revert the commit rather than track individual changes. So let's just make sure we can find the PRs we need to revert.

[zakkak]

@gsmet OK

The tests in my CI run look good. Please merge once the Quarkus CI is happy as well :)

[gsmet]

I will split some of the work into a separate PR as some fixes need to stay after reverting this.

[gsmet]

Note for future selves: when dropping Mandrel 20.3 support, please only revert the last commit!