Bump golang to 1.16.4 and alpine to 3.13.5, updated deps

[zigmund]

Updated deps, golang and alpine to fix some known CVEs found in current outdated libs and images.

Before:

$ trivy mintel/dex-k8s-authenticator:1.4.0           
2021-05-27T13:17:17.164+0600    INFO    Detected OS: alpine
2021-05-27T13:17:17.165+0600    INFO    Detecting Alpine vulnerabilities...
2021-05-27T13:17:17.173+0600    INFO    Number of PL dependency files: 1
2021-05-27T13:17:17.173+0600    INFO    Detecting gobinary vulnerabilities...
2021-05-27T13:17:17.175+0600    WARN    This OS version is no longer supported by the distribution: alpine 3.10.3
2021-05-27T13:17:17.176+0600    WARN    The vulnerability detection may be insufficient because security updates are not provided

mintel/dex-k8s-authenticator:1.4.0 (alpine 3.10.3)
==================================================
Total: 27 (UNKNOWN: 0, LOW: 3, MEDIUM: 13, HIGH: 11, CRITICAL: 0)
...

app/bin/dex-k8s-authenticator
=============================
Total: 5 (UNKNOWN: 2, LOW: 0, MEDIUM: 1, HIGH: 2, CRITICAL: 0)
...

After:

$ trivy dex-k8s-authenticator:bump-golang1.16.4-alpine3.13.5 
2021-05-27T13:48:15.318+0600    INFO    Detected OS: alpine
2021-05-27T13:48:15.318+0600    INFO    Detecting Alpine vulnerabilities...
2021-05-27T13:48:15.320+0600    INFO    Number of PL dependency files: 1
2021-05-27T13:48:15.320+0600    INFO    Detecting gobinary vulnerabilities...

dex-k8s-authenticator:bump-golang1.16.4-alpine3.13.5 (alpine 3.13.5)
====================================================================
Total: 0 (UNKNOWN: 0, LOW: 0, MEDIUM: 0, HIGH: 0, CRITICAL: 0)

app/bin/dex-k8s-authenticator
=============================
Total: 0 (UNKNOWN: 0, LOW: 0, MEDIUM: 0, HIGH: 0, CRITICAL: 0)

[nabadger]

Thanks :)