"which solr version is compatile with moodle 3.6.6" ...
Sorry ... cannot ... first, you've kinda piggy backed in a discussion without providing any info about your system ... which may/may NOT be the same as in this thread.
2nd ... know this may not the answer you want to hear but ... philosophy of admin of public facing dynamic app like Moodle ...
IMHO ... it's becoming increasing difficult to run any web server today ... many attack vectors. One way to reduce risk is reduce the attack surface ... and basically that means don't run anything you can't understand which means can't control. Solr search requires ... apache solr, which requires java (just today, an update to java6 and java7 for CentOS servers).
IF an update to apache solr or java whacks moodle's solr search is solr search really worth the trouble considering there is an option for 'simplesearch' in Moodle.
I've made my choice ... being a simple minded person ... Moodle's 'simplesearch' is ok by me. BTW, all of my 'clients' when they asked about search definitely did not want site search ... maybe only course search (site found too much).