ci: add ENVOY_VERSIONS file for nightly int tests

This file will be sourced by nightly integration tests, so must exist on
both CE and Ent regardless of whether CE is actively released.

envoyextensions/xdscommon/ENVOY_VERSIONS

@@ -0,0 +1,14 @@
+# This file represents the canonical list of supported Envoy versions for this version of Consul.
+#
+# Every line must contain a valid version number in the format "x.y.z" where x, y, and z are integers.
+# All other lines must be comments beginning with a "#", or a blank line.
+#
+# Every prior "minor" version for a given "major" (x.y) version is implicitly supported unless excluded by
+# `xdscommon.UnsupportedEnvoyVersions`. For example, 1.28.3 implies support for 1.28.0, 1.28.1, and 1.28.2.
+#
+# See https://www.consul.io/docs/connect/proxies/envoy#supported-versions for more information on Consul's Envoy
+# version support.
+1.27.6
+1.26.8
+1.25.11
+1.24.12