chore: update Go toolchain to v1.15.8

[mboersma]

Reason for Change:

See https://github.com/deis/docker-go-dev/releases/tag/v1.29.2
and https://github.com/golang/go/issues?q=milestone%3AGo1.15.8+label%3ACherryPickApproved

Issue Fixed:

Credit Where Due:

Does this change contain code from or inspired by another project?

• No
• Yes

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[codecov]

Codecov Report

Merging #4307 (796462e) into master (edeee21) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4307   +/-   ##
=======================================
  Coverage   72.03%   72.03%           
=======================================
  Files         141      141           
  Lines       21633    21633           
=======================================
  Hits        15584    15584           
  Misses       5099     5099           
  Partials      950      950           

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[jackfrancis]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis, mboersma

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