Release v13.21.0

Quite a few changes in gitlab-shell we need to flush out.

This release is also complicated by the fact that we never really used gitlab-shell v13.20.0 - it was bumped here: gitlab!65625 (merged) but almost immediately superceded by v13.19.1 via a security release. So in practice, most people will be getting 13.20.0 changes in this bump as well.

Here's the output of git log --first-parent v13.20.0..main --format=oneline:

a83f8b31982222ad64b23a47ee6d092042cf9f1a Merge branch 'bump/go-1.16' into 'main'
11b8edd69179aaa01ccaf11723aaa47b86f18de2 Merge branch 'test/go-1.17' into 'main'
121b28daf2043f3f8619db69bbff03b5eb59eef2 Merge branch 'verify/cafile' into 'main'
51e7ed1be98c7cdc65078b604e2c46cd551d3c2e Merge branch 'update/newclientopts' into 'main'
fa9e9ad01c410fc47f751e7aef2202928ac1a3ee Merge branch 'bump/go1.15' into 'main'
7b44ce1d4a0716d27acabb4f826eb5613dade082 Merge branch '499-use-labkit-logging-initialize' into 'main'
df0138b5f0f8f998a832c6c063128f56d1953604 Merge branch 'security-300265' into 'main'
00735e0bbf51e28bcec5086d9d0f62999d19d2c5 Merge branch '477-remove-logrus-tests' into 'main'
b7edd7dd9f957c6b14d3bfa4407aca9ddfbe4f52 Merge branch 'id-ctx-for-auth-check' into 'main'
f9e7ffda68192d24ff26f0d5ff7fe70e376c32f2 Merge branch 'sh-update-go-proxyproto' into 'main'
82a2153dcc093a99445aedd3734fe7ef868e3a4b Merge branch 'sh-fix-gofmt' into 'main'
1eadd3a61f3955354b4041fd284ba50e2a17864a Merge branch 'id-switch-logging-to-labkit' into 'main'
05ae5680633c1de1cc369c592f3bbcce13ee7375 Merge branch '521-test-connection-dot-go' into 'main'
a8b2088d6d40e365445fcf4bea5183f83e31cc51 Merge branch 'id-prometheus-metrics-for-http' into 'main'
50da611814d256c77e689977265ec7e07633a4dc Merge branch 'igorwwwwwwwwwwwwwwwwwwww-main-patch-84756' into 'main'
8c932b0d5a698ea6b9a770be8f8c6ecc58b38727 Merge branch 'id-kubernetes-probes' into 'main'
60a91e977cb76e2cc49faeeac134640dbf2e2e6c Merge branch 'id-cancelable-sshd' into 'main'
d3711d8d7e781dbff01d8ae5c7a1d5b800c5c8a2 Merge branch 'id-refactor-test-helper' into 'main'
b16c323cc832be3fa3b4bcedcc5a90bfcdc6841a Merge branch 'change_log_format' into 'main'

The user-facing stuff is to be found in the CHANGELOG file.

I also added the entries for v13.19.1 and v13.18.1, which were missed when the security release was made.