Clean up renovate config around Spring Boot 2 package rules

[trask]

Implements #15318 (review)

but also a bit of cleanup

[laurit]

In #15318 (review) I meant using

opentelemetry-java-instrumentation/smoke-tests-otel-starter/spring-boot-2/build.gradle.kts

Lines 16 to 17 in ed38281

	val testLatestDeps = gradle.startParameter.projectProperties["testLatestDeps"] == "true"
	implementation(platform("org.springframework.boot:spring-boot-dependencies:" + if (testLatestDeps) "2.+" else "2.6.15"))

renovate doesn't seem to update the versions in this construct + it also tests latest