Update to golangci-lint v1.39.x (#57)

The currently latest `golangci-lint` version 1.39.0 [1] introduced new
linters and updated supported ones:

1. predeclared [2] (v1.35.0 [3]) - Checks for definitions and
   declarations that shadows one of Go's pre-declared identifiers [4].
   This linter is disabled by default, but is enabled for this template
   to help to prevent shadowed identifiers.
2. interfacer [5] (v1.38.0 [6]) - Has been deprecated [7] and removed
   from this template.
3. scopelint [8] (v1.39.0 [9]) - Has been deprecated [10] and replaced
   by exportloopref [11].
   The `exportloopref` linter is disabled by default, but is enabled for
   this template to help to catch loop variable bugs.

[1]: https://github.com/golangci/golangci-lint/releases/tag/v1.39.0
[2]: https://github.com/nishanths/predeclared
[3]: https://github.com/golangci/golangci-lint/releases/tag/v1.35.0
[4]: https://golang.org/ref/spec#Predeclared_identifiers
[5]: https://github.com/mvdan/interfacer
[6]: https://github.com/golangci/golangci-lint/releases/tag/v1.38.0
[7]: golangci/golangci-lint#1755
[8]: https://github.com/kyoh86/scopelint
[9]: https://github.com/golangci/golangci-lint/releases/tag/v1.39.0
[10]: golangci/golangci-lint#1819
[11]: https://github.com/kyoh86/exportloopref

Closes GH-56

.github/workflows/ci.yml

@@ -51,7 +51,7 @@ jobs:
         # See https://github.com/golangci/golangci-lint-action for more details.
         uses: golangci/[email protected]
         with:
-          version: v1.32
+          version: v1.39
   test:
     strategy:
       matrix:

.golangci.yml

@@ -33,6 +33,7 @@ linters:
     - dupl
     - errcheck
     - exhaustive
+    - exportloopref
     - gochecknoinits
     - goconst
     - gocritic
@@ -49,13 +50,11 @@ linters:
     - gosimple
     - govet
     - ineffassign
-    - interfacer
     - lll
     - misspell
     - nakedret
     - noctx
     - nolintlint
-    - scopelint
     - staticcheck
     - structcheck
     - stylecheck
@@ -147,4 +146,4 @@ run:
   deadline: 15m
 
 service:
-  golangci-lint-version: 1.32.x
+  golangci-lint-version: 1.39.x