Track the incompatibilities introduced in Solr 9

This is a running list and will be updated overtime.

- Solr modules have moved from contrib/ to modules/ (SOLR-15917)
- This may not be an issue. We can double-add the modules in an old and new way:
- <=8: Continue using the sharedLib strategy (adding paths to /contrib//lib and dist/)
- \>=9: Use the new SOLR_MODULES envVar introduced in SOLR-15914
- The Prometheus Exporter is no longer a module (SOLR-15954)
- This messes up the default paths that we use for the exporter command and default metrics config. (since they will be different from <=8 and >=9.
- One option for the exporter command is to backport the PATH changes to the docker-solr repo, so that solr-exporter is added to the path for all versions of Solr. That way the operator can merely use the command: `solr-exporter`
- The default config is a non-issue in Solr 9+ because it is added to the classpath, and the classpath option is used by default. (SOLR-14957) However we still have to pass the option because it is required in Solr 8-

Start with the Solr module paths, SOLR_MODULES, solr-exporter command, and default metrics config described in the issue, comparing Solr <=8 with Solr >=9. Confirm which incompatibilities still need handling and update the running list so exporter paths and the required Solr 8 config option are correct for each version.

Written by the indexing model from the issue text.