Changes

[eclipse/xtext#1431] Tweak resource constraints
Tests have shown that at most 2 executors will usually be available at 
most and these resource configurations seem to make optimal usage of 
available resources.
- Container 'jnlp' is working with 0.4Gi memory (default is 0.5Gi) and 
0.2 cpu (this is the default, lower value leads to slower agent 
provisioning)
- Build container can use at max 3.6Gi memory and 1.0 cpu. Higher values 
will lead to only 1 executor.
Signed-off-by: Karsten Thoms <karsten.thoms@itemis.de>

[eclipse/xtext#1384] Convert to declarative pipeline
Also add slack notification
Signed-off-by: Karsten Thoms <karsten.thoms@itemis.de>

[#138] Removed duplicate settings.xml provisioning
The config file id for settings.xml was invalid. Since the settings.xml 
is already provided by volume mount in the pod configuration, additional 
provisioning was obsolete anyway.
Signed-off-by: Karsten Thoms <karsten.thoms@itemis.de>

[eclipse/xtext#1489] update to gradle wrapper 5.5
Signed-off-by: Christian Dietrich <christian.dietrich@itemis.de>

[eclipse/xtext#1471] Add releng job for cleaning up nightly
Signed-off-by: Nico Prediger <mail@nicoprediger.de> Also-by: Karsten
Thoms <karsten.thoms@itemis.de>

[eclipse/xtext#1500] Improve Slack notification
- Reduce envName to known environments per Jenkinsfile type:
 - Jenkinsfile: remove CBI
 - CBI.Jenkinsfile: remove CBI & TF
 - CBI.Jenkinsfile: rename JIPP => JIRO
- Mark FAILURE, ABORTED, NOT_BUILD red
- Report FIXED only when the previous build was finished not with 
SUCCESS
Signed-off-by: Karsten Thoms <karsten.thoms@itemis.de>

[eclipse/xtext#1471] Add sign-and-deploy Jenkinsfile
This pipeline replaces the xtext-snapshots and common-deploy jobs from 
JIPP. It signs Xtext artifacts with the Eclipse Signing Service and with 
gpg for deployment for OSSRH. Artifacts are uploaded to OSSRH. The
resulting p2 repository is zipped and uploaded to Eclipse projects 
storage and unpacked to the Xtext update locations.
Signed-off-by: Karsten Thoms <karsten.thoms@itemis.de>