Dependency: update PGV, opencensus-proto, nghttp2, msgpack-c, bazel-gazelle & cleanup (#7787)

Update dependencies:

* PGV (changes)
* switch opencensus-proto to 0.2.1 release (changes) /cc @kyessenov
* nghttp2 1.39.1 (release notes 1.39.0, 1.39.1)
* rules_go 0.19.1 (0.19.0 release notes) this is breaking the API only. Reverting to master and will sort out later.
* msgpack-c 3.2.0 (release notes)
* bazel-gazelle 0.18.1 (0.18.0 release notes)
* comment cleanup (remove uneccesary todo and add dates to commits)
* removed unused old backward dependency build file

Risk Level: Medium
Testing: bazel test //test/...

Signed-off-by: Michael Payne <michael@sooper.org>

Mirrored from https://github.com/envoyproxy/envoy @ 07267190c6d8b88ff55c5b9fe607221f7cdcca99

bazel/repository_locations.bzl

@@ -4,11 +4,11 @@ BAZEL_SKYLIB_SHA256 = "2ef429f5d7ce7111263289644d233707dba35e39696377ebab8b0bc70
 GOGOPROTO_RELEASE = "1.2.1"
 GOGOPROTO_SHA256 = "99e423905ba8921e86817607a5294ffeedb66fdd4a85efce5eb2848f715fdb3a"
 
-OPENCENSUS_PROTO_GIT_SHA = "d5d80953a8c2ff4633087af6933cd152678434bb"  # Mar 18, 2019
-OPENCENSUS_PROTO_SHA256 = "a4e87a1da21d1b3a16674332c3ee6e2689d52f3532e2ff8cb4a626c8bcdabcfc"
+OPENCENSUS_PROTO_RELEASE = "0.2.1"
+OPENCENSUS_PROTO_SHA256 = "bfcefa6093fc2ecdf5c9effea86e6982d0d6f9a5178b17fcf73a62e0f3fb43d0"
 
-PGV_GIT_SHA = "dd90514d96e35023ed20201f349542b0bc64461d"
-PGV_SHA256 = "f7d67677fbec5b0f561f6ee6788223fb535d3864b53a28b6678e319f5d1f4f25"
+PGV_GIT_SHA = "2feaabb13a5d697b80fcb938c0ce37b24c9381ee"  # Jul 26, 2018
+PGV_SHA256 = "ddefe3dcbb25d68a2e5dfea67d19c060959c2aecc782802bd4c1a5811d44dd45"
 
 GOOGLEAPIS_GIT_SHA = "be480e391cc88a75cf2a81960ef79c80d5012068"  # Jul 24, 2019
 GOOGLEAPIS_SHA = "c1969e5b72eab6d9b6cfcff748e45ba57294aeea1d96fd04cd081995de0605c2"
@@ -46,8 +46,8 @@ REPOSITORY_LOCATIONS = dict(
     ),
     opencensus_proto = dict(
         sha256 = OPENCENSUS_PROTO_SHA256,
-        strip_prefix = "opencensus-proto-" + OPENCENSUS_PROTO_GIT_SHA + "/src",
-        urls = ["https://github.com/census-instrumentation/opencensus-proto/archive/" + OPENCENSUS_PROTO_GIT_SHA + ".tar.gz"],
+        strip_prefix = "opencensus-proto-" + OPENCENSUS_PROTO_RELEASE + "/src",
+        urls = ["https://github.com/census-instrumentation/opencensus-proto/archive/v" + OPENCENSUS_PROTO_RELEASE + ".tar.gz"],
     ),
     kafka_source = dict(
         sha256 = KAFKA_SOURCE_SHA,