Fix compilation issue by using Instant API

Calling `getLastUpdatedOn()` on a `Change` object now returns an
`Instant` rather than a `JavaUtilDate`.

Using .getTime() on it fails compilation for a `cannot find symbol`
error.

Use instead the `toEpochMilli()` to return the number of milliseconds
elapsed since epoch.

Bug: Issue 15833
Change-Id: I84f53a89e67c295ec13051b3f5e6e4252efa54fd

src/main/java/com/googlesource/gerrit/modules/gitrefsfilter/ChangesTsCache.java

diff --git a/src/main/java/com/googlesource/gerrit/modules/gitrefsfilter/ChangesTsCache.java b/src/main/java/com/googlesource/gerrit/modules/gitrefsfilter/ChangesTsCache.java
index cfd7a0f..c371985 100644
--- a/src/main/java/com/googlesource/gerrit/modules/gitrefsfilter/ChangesTsCache.java
+++ b/src/main/java/com/googlesource/gerrit/modules/gitrefsfilter/ChangesTsCache.java
@@ -53,7 +53,7 @@
             .createChecked(key.repo(), key.project(), key.changeId(), key.changeRevision())
             .getChange()
             .getLastUpdatedOn()
-            .getTime();
+            .toEpochMilli();
       } catch (NoSuchChangeException e) {
         logger.atFine().withCause(e).log(
             "Change %d does not exist: returning zero epoch", key.changeId());