Deprecate sniffs being removed in VIPCS 3.0

[GaryJones]

What problem would the enhancement address for VIP?

To provide a friendlier developer experience, we can formally deprecate the sniffs in 2.x that we know will be removed in 3.x.

See:

• Remove WordPressVIPMinimum.Variables.VariableAnalysis #496
• Remove ZoninatorSniff #612
• Remove BatcacheWhitelistedParamsSniff #613

Describe the solution you'd like

Action plan suggestion:

• Deprecate the sniff in the next release (2.x) by:
  • Adding a deprecation warning to the sniff which will inform people to remove the sniff reference from their own custom ruleset.
  • Silencing all messages from the sniff from within the VIPMinimum ruleset, effectively removing the sniff already for people using the standard rulesets.
• Remove the actual sniff in the next major, i.e. 3.0.0.

[jrfnl]

Maybe just add a note in the changelog (soft deprecation) of 2.3.4 ?

[jrfnl]

Closing as fixed via soft deprecation in the 2.3.4 release.

(issue didn't close automatically as the branch was merged to master, not develop)