Update default ruleset and documentation links

[tomekduda]

Is your feature request related to a problem? Please describe.

Yes. I use default ruleset's override based on apex_ruleset as linked in readme. I noticed the plugin doesn't report Database.query(), Database.insert() etc variants of SOQL/DML in a loop. Ruleset wasn't modified since March 2020.

If you run PMD 6.49.0 from commandline - it emits warnings.

WARNING: Discontinue using Rule name category/apex/performance.xml/AvoidSoqlInLoops as it is scheduled for removal from PMD.
PMD 7.0.0 will remove support for this Rule. (...) 
AvoidSoslInLoops (...) 
AvoidDmlStatementsInLoops (...) 
VariableNamingConventions (...)

replacing the 3 performance rules with category/apex/performance.xml/OperationWithLimitsInLoop correctly detects the Database.query()

Describe the solution you'd like

• update your apex_ruleset.xml. Maybe use quickstart.xml, the "ruleset.xml" in same directory is deprecated
• update readme. Current one has "PMD System Requirements" and "Apex Ruleset Reference" links pointing to 6.11. Either pin 6.49.0 or just https://pmd.github.io/latest/pmd_rules_apex.html ?

Additional context
I'd like this file to count as 4 performance problems even on vanilla installation

public with sharing class PmdDemo {
    public void thisWorksAsExpected() {
        for(Integer i = 0; i < 10; ++i){
            List<Account> accs = [SELECT Id FROM Account WITH SECURITY_ENFORCED LIMIT 10];
            insert new Task();
        }
    }

    public void makeThisWorkToo() {
        for(Integer i = 0; i < 10; ++i){
            List<Account> accs = Database.query('SELECT Id FROM Account WITH SECURITY_ENFORCED LIMIT 10');
            Database.insert(new Task());
        }
    }
}