[keycloak-dev] Update to 3.4 release dates

Update release dates: * 16 October - Feature freeze! Only bug fixes after this point please * 30 October - 3.4.0.CR1 * 6 November - 3.4.0.Final * 20 November - 3.4.1.Final * Mid December - master switches to 4.x Community contribution deadlines for 3.x: * 12 October - deadline for feature requests and bigger enhancements * 26 October - deadline for small enhancements * 1 November - deadline for bug fixes Anything contributed after these dates will most likely not be reviewed/merged until 4.x