drop support for Go 1.15

[marten-seemann]

In preparation for the upcoming Go 1.17 release.

[codecov]

Codecov Report

Merging #3247 (68a1d81) into master (761c0fa) will increase coverage by 0.03%.
The diff coverage is 100.00%.

❗ Current head 68a1d81 differs from pull request most recent head 68e468a. Consider uploading reports for the commit 68e468a to get more accurate results

@@            Coverage Diff             @@
##           master    #3247      +/-   ##
==========================================
+ Coverage   85.43%   85.47%   +0.03%     
==========================================
  Files         133      131       -2     
  Lines        9803     9791      -12     
==========================================
- Hits         8375     8368       -7     
+ Misses       1053     1048       -5     
  Partials      375      375              

Impacted Files	Coverage Δ
internal/qerr/errors.go	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 761c0fa...68e468a. Read the comment docs.