Rely on conventions to configure source and encoding of aggregated Javadoc task

[wilkinsona]

source is hardcoded as 1.8 and encoding as UTF-8:

spring-boot/spring-boot-project/spring-boot-docs/build.gradle

Lines 194 to 204 in b2e63db

	options {
	author = true
	docTitle = "Spring Boot ${project.version} API"
	encoding = "UTF-8"
	memberLevel = "protected"
	outputLevel = "quiet"
	source = "1.8"
	splitIndex = true
	use = true
	windowTitle = "Spring Boot ${project.version} API"
	}

In order to avoid problems like this one, the configured conventions should be used instead.