hardcode buildTimestamp for reproducibility

* I did that because there are testcases which failed when removing the buildTimestamp completely. Also there is code reading it. To stay backwards compatible we return a fixed date.
* if this should be removed completely then more places needed to be adjusted, which I tried to avoid here to keep it simple
diff --git a/freemarker-core/src/main/resource-templates/freemarker/version.properties b/freemarker-core/src/main/resource-templates/freemarker/version.properties
index 95cd4ef..6c4d072 100644
--- a/freemarker-core/src/main/resource-templates/freemarker/version.properties
+++ b/freemarker-core/src/main/resource-templates/freemarker/version.properties
@@ -95,7 +95,10 @@
 versionForMf=2.3.32.97
 
 # The date of the build.
-# This should be automatically filled by the building tool (Ant).
-buildTimestamp=@timestampNice@
+# For reproducible builds we set it to a fixed date.
+# Although this is rather meaningless now
+# it is kept for backwards compatibility (tests and Version.java)
+# until decided to drop it completely
+buildTimestamp=1980-02-01T00:00:00Z
 
 isGAECompliant=true