LOGBACK-927 Update logback-access Common Log Format to use MMM rather than MM for month

LOGBACK-927

* Updates the default Common Log Format date for logback-access to include the 3-character abbreviated month, rather than digits. This format should now conform to the CLF format [defined by apache httpd](https://httpd.apache.org/docs/trunk/logs.html#common)
* Updates issues in the documentation for the logback-access PatternLayout:
    * Default date format is now defined as CLF (rather than ISO8601)
    * Removed references to magic strings ```ABSOLUTE```, ```DATE```, which aren't valid
    * Updated date specifiers to be %t rather than %d