Fix duplicate ExitSpan, Maybe this will cause some error message not recorded. because "feign load balance" mechanism will retry execute. (#5667)

* resttemplate plugin concurrency scenario data mistake problem 

Co-authored-by: 吴晟 Wu Sheng <wu.sheng@foxmail.com>

Co-authored-by: 吴晟 Wu Sheng <wu.sheng@foxmail.com>

Implicit use of the platform default charset, which can result in differing behaviour between JVM executions or incorrect behavior if the encoding of the data source doesn't match expectations.
Co-authored-by: Nzhang-wei <pknfe@outlook.com>
Co-authored-by: 吴晟 Wu Sheng <wu.sheng@foxmail.com>

Co-authored-by: 吴晟 Wu Sheng <wu.sheng@foxmail.com>

* Fix JdkObsolete issue

* Fix code style
Co-authored-by: 吴晟 Wu Sheng <wu.sheng@foxmail.com>

* Upgrade dependencies and make the JDK11 compiling passed.

* Update license and fix license check file.

* Add CI for JDK11 compiling.

* Update doc.

* modify witness class

* modify witness class & add test case

* replace DB_STATEMENT to DB_BIND_VARIABLES & remove executeMethod prefix

* fix ci
Co-authored-by: 吴晟 Wu Sheng <wu.sheng@foxmail.com>

* Move spring webflux into optional plugin

* Fix compile check

* Fix check style

* Adjust code

* add docs

* Update docs/en/setup/service-agent/java-agent/README.md
Co-authored-by: 吴晟 Wu Sheng <wu.sheng@foxmail.com>

Support @KafkaPollAndInvoke annotation to replace @Trace when just using the Kafka scenario agent plugin (#5304)

use the  method httpMethod.setRequestHeader to set the new skywalking headers,so that it can override the exits headers if there is any
Co-authored-by: 吴晟 Wu Sheng <wu.sheng@foxmail.com>

* [maven-release-plugin] prepare release v8.1.0

* [maven-release-plugin] prepare for next development iteration