Remove obsolete options 'UseConcMarkSweepGC'

[Ashita]

The option UseConcMarkSweepGC as been deprecated since version 9 and ignored until version 14 of the openJDK ( https://openjdk.java.net/jeps/363 ) and it has now been deleted in v15.

It now generate an error at execution and prevent the application to start :

Unrecognized VM option 'UseConcMarkSweepGC'
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.

I think it should be removed.

[vakuum]

Maybe we can remove all GC_TUNE settings from solr.in.sh and solr.in.cmd?

• https://github.com/sunspot/sunspot/blob/v2.5.0/sunspot_solr/solr/bin/solr.in.sh#L36
• https://github.com/sunspot/sunspot/blob/v2.5.0/sunspot_solr/solr/bin/solr.in.cmd#L34

These settings have already caused some trouble in the past:

• https://github.com/sunspot/sunspot/commits/v2.5.0/sunspot_solr/solr/bin/solr.in.sh
• https://github.com/sunspot/sunspot/commits/v2.5.0/sunspot_solr/solr/bin/solr.in.cmd

For local development these settings are not necessary and the README.md already states that sunspot_solr should not be used in production.

[vakuum]

The GC_TUNE settings are also disabled in the current upstream version of Solr:

• https://github.com/apache/lucene-solr/blob/branch_8_8/solr/bin/solr.in.sh#L52
• https://github.com/apache/lucene-solr/blob/branch_8_8/solr/bin/solr.in.cmd#L35
• apache/lucene-solr@430f059

Comment from https://issues.apache.org/jira/browse/SOLR-13394: "This stuff is magic voodoo without commentary". 🤩

[mlh758]

In #993 there is some work being done to have the library work better with Solr 8. I think the plan is to remove the bundled solr deployment from this gem in what would end up being a major version release. For now I would really recommend you use a docker image or something for developing against instead of the bundled library. Solr 5 probably isn't what you want anyway.