I have the following settings in my build.gradle file:

// Jenkins 1.580+ still uses Java 7, not Java 8.
sourceCompatibility = 1.7
targetCompatibility = 1.7

then I run "gradle jpi" on this plugin
note: the default Java available to Gradle during the build was Java 8.

then I upload the plugin to a Jenkins server that is running on Java 7.

I suspect the plugin build ignored "targetCompatibility" because the plugin execution failed in Jenkins:

ava.lang.VerifyError: Expecting a stackmap frame at branch target 7
Exception Details:
Location:
com/codepine/api/testrail/model/Project.____isAssignableFrom0(Ljava/lang/Class;)Z @0: ldc
Reason:
Expected stackmap frame at this location.
Bytecode:
0000000: 12bc 2ab6 00c1 ac12 c4ac
Exception Handler Table:
bci [0, 7] => handler: 7

at java.lang.Class.getDeclaredFields0(Native Method)
at java.lang.Class.privateGetDeclaredFields(Class.java:2499)
at java.lang.Class.getDeclaredFields(Class.java:1811)
at com.fasterxml.jackson.databind.introspect.AnnotatedClass._findFields(AnnotatedClass.java:683)
at com.fasterxml.jackson.databind.introspect.AnnotatedClass.resolveFields(AnnotatedClass.java:470)
at com.fasterxml.jackson.databind.introspect.AnnotatedClass.fields(AnnotatedClass.java:282)
at com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector._addFields(POJOPropertiesCollector.java:380)
at com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector.collect(POJOPropertiesCollector.java:233)
at com.fasterxml.jackson.databind.introspect.BasicClassIntrospector.collectProperties(BasicClassIntrospector.java:142)
at com.fasterxml.jackson.databind.introspect.BasicClassIntrospector.forDeserialization(BasicClassIntrospector.java:81)
at com.fasterxml.jackson.databind.introspect.BasicClassIntrospector.forDeserialization(BasicClassIntrospector.java:11)
at com.fasterxml.jackson.databind.DeserializationConfig.introspect(DeserializationConfig.java:547)
at com.fasterxml.jackson.databind.deser.DeserializerCache._createDeserializer(DeserializerCache.java:325)
at com.fasterxml.jackson.databind.deser.DeserializerCache._createAndCache2(DeserializerCache.java:263)
at com.fasterxml.jackson.databind.deser.DeserializerCache._createAndCacheValueDeserializer(DeserializerCache.java:243)
at com.fasterxml.jackson.databind.deser.DeserializerCache.findValueDeserializer(DeserializerCache.java:144)
at com.fasterxml.jackson.databind.DeserializationContext.findContextualValueDeserializer(DeserializationContext.java:366)
at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.createContextual(CollectionDeserializer.java:150)
at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.createContextual(CollectionDeserializer.java:23)
at com.fasterxml.jackson.databind.DeserializationContext.handleSecondaryContextualization(DeserializationContext.java:572)
at com.fasterxml.jackson.databind.DeserializationContext.findRootValueDeserializer(DeserializationContext.java:386)
at com.fasterxml.jackson.databind.ObjectMapper._findRootDeserializer(ObjectMapper.java:3095)
at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2989)
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2151)
at com.codepine.api.testrail.Request.execute(Request.java:164)
at org.jenkinsci.plugin.trp.TestRailClient.getProjectId(TestRailClient.java:104)
at org.jenkinsci.plugin.trp.BuildRunListener.processBuildCompletedNotification(BuildRunListener.java:59)
at org.jenkinsci.plugin.trp.BuildRunListener.onCompleted(BuildRunListener.java:39)
at hudson.model.listeners.RunListener.fireCompleted(RunListener.java:201)
at hudson.model.Run.execute(Run.java:1786)
at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:537)
at hudson.model.ResourceController.execute(ResourceController.java:98)
at hudson.model.Executor.run(Executor.java:381)