Prepare release 1.6.0

$ git log --oneline --no-merges v1.5.0..HEAD
9d7a427 Update jgit to 2d8777536 (servlet-4 branch)
946a41d Bump minimum Java version to 17 and add Java 21 build
98a9b85 README: Point link to gerrit's bugtracker
859617c Documentation - API Reference
5371b1f Add committeroneline template to gitiles LogDetail.soy
a37b23e Bump JGit to d5cc102e7 (stable-6.10)
30851aa VisibilityChecker: Remove deprecated variant of isReachableFrom
6454234 VisibilityChecker#isReachableFrom: Remove unused param and exceptions
9b81f87 VisibilityCache/Checker: Use streams for the reachability starters
ace68b4 Document enabling edit links by setting gerritUrl
04b5095 Fix source, log and blame URLs not showing in Markdown doc footers
77c2325 Format sources using google-java-format 1.24.0

Change-Id: I69ead0ac4b5901d161813d9194d2690a2ac09fff

Author: msohn

version.bzl

@@ -3,4 +3,4 @@
 # Used by :install and :deploy when talking to the destination repository.
 # Project uses semantic versioning described at:
 # https://semver.org
-GITILES_VERSION = "1.5.0"
+GITILES_VERSION = "1.6.0"