Change orbit repo to be 2020-12 repo

Change-Id: I07858240bfe07bf22df9c2f55bc8a99429cef0af
Reviewed-on: https://git.eclipse.org/r/c/linuxtools/org.eclipse.linuxtools/+/173047
Tested-by: Linux Tools Bot <[email protected]>
Reviewed-by: Jeff Johnston <[email protected]>

Author: jjohnstn

releng/org.eclipse.linuxtools.target/linuxtools-e4.18.target

@@ -56,7 +56,7 @@
 <unit id="org.objectweb.asm.tree" version="9.0.0.v20201001-1419"/>
 <unit id="org.objectweb.asm.util" version="9.0.0.v20201001-1419"/>
 <unit id="org.slf4j.api" version="1.7.30.v20200204-2150"/>
-<repository location="https://download.eclipse.org/tools/orbit/downloads/drops/S20201124213957/repository"/>
+<repository location="https://download.eclipse.org/tools/orbit/downloads/2020-12"/>
 </location>
 <location includeAllPlatforms="false" includeConfigurePhase="false" includeMode="planner" includeSource="true" type="InstallableUnit">
 <unit id="org.eclipse.jdt.annotation" version="0.0.0"/>