Add sniff detecting deprecated $CFG properties.

[paulholden]

Adds a new moodle.PHP.DeprecatedCFGProperties sniff that warns on any use of removed-but-aliased $CFG properties. Ships with an entry for $CFG->httpswwwroot (removed in Moodle 3.4, still aliased for BC), suggesting $CFG->wwwroot as the replacement.

Detects both bare-code accesses and references inside double-quoted strings / heredocs, and only fires when the variable is exactly $CFG.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.09%. Comparing base (b60df78) to head (318caac).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
moodle/Sniffs/PHP/DeprecatedCFGPropertiesSniff.php	83.33%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #233      +/-   ##
============================================
- Coverage     98.27%   98.09%   -0.18%     
- Complexity      994     1008      +14     
============================================
  Files            42       43       +1     
  Lines          2961     2997      +36     
============================================
+ Hits           2910     2940      +30     
- Misses           51       57       +6     

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