This change allows for setting the `minimum_supported_version` property for all three `WP.Deprecated...` sniffs in one go, either via the command-line or by setting a config variable in a custom ruleset.

As this config variable is no longer directly related to a sniff, I've chosen to make the name slightly more specific/descriptive: `minimum_supported_wp_version` <= note the `_wp_`.

Usage:
```bash
phpcs . --standard=WordPress --runtime-set minimum_supported_wp_version 4.5
```

or in a ruleset:
```xml
<config name="minimum_supported_wp_version" value="4.5"/>
```

The above one liner can replace the custom property which would otherwise need to be set for all three sniffs individually.

Overrule order:
1. Value passed via the command line
2. `<config>` property in the ruleset
3. Individual sniff `<property>`'s set via the ruleset
4. Default value

Simplest way to test this is by running PHPCS over the `WP.DeprecatedParameters` sniff unit tests:
```bash
phpcs -p -s ./WordPress/Tests/DeprecatedParametersUnitTest.inc --standard=WordPress --sniffs=WordPress.WP.DeprecatedParameters

phpcs -p -s ./WordPress/Tests/DeprecatedParametersUnitTest.inc --standard=WordPress --sniffs=WordPress.WP.DeprecatedParameters --runtime-set minimum_supported_wp_version 4.8

phpcs -p -s ./WordPress/Tests/DeprecatedParametersUnitTest.inc --standard=WordPress --sniffs=WordPress.WP.DeprecatedParameters --runtime-set minimum_supported_wp_version 3.8
```

Additional notes:
* I've moved the `$custom_test_class_whitelist` property up in the `WordPress\Sniff` class, so both `public` properties are now at the top of the file.