golangci-lint: update config file to conform with 1.61.0 changes

A series of changes have made the current .golangci.yml configuration
file non-conformant with latest updates, causing warnings and errors
when attempting to run the tool on the plugin.

This commit fixes all the reported issues with the configurtion,
removing dead checkers and updating the deprecated/removed options in
the config file.

Author: lbajolet-hashicorp

.golangci.yml

@@ -20,6 +20,9 @@ issues:
     - linters:
         - lll
       source: "^//go:generate "
+    - linters:
+        - errcheck
+      path: ".*_test.go"
 
   # Maximum issues count per one linter. Set to 0 to disable. Default is 50.
   max-issues-per-linter: 0
@@ -30,7 +33,6 @@ issues:
 linters:
   disable-all: true
   enable:
-    - deadcode
     - errcheck
     - goimports
     - gosimple
@@ -39,7 +41,6 @@ linters:
     - staticcheck
     - unconvert
     - unused
-    - varcheck
   fast: true
 
 # options for analysis running
@@ -76,7 +77,7 @@ run:
   # won't be reported. Default value is empty list, but there is
   # no need to include all autogenerated files, we confidently recognize
   # autogenerated files. If it's not please let us know.
-  skip-files:
+  exclude-files:
     - ".*\\.hcl2spec\\.go$"
   #  - lib/bad.go
 
@@ -93,7 +94,7 @@ run:
 # output configuration options
 output:
   # colored-line-number|line-number|json|tab|checkstyle|code-climate, default is "colored-line-number"
-  format: colored-line-number
+  formats: colored-line-number
 
   # print lines of code with issue, default is true
   print-issued-lines: true
@@ -119,7 +120,7 @@ linters-settings:
     # [deprecated] comma-separated list of pairs of the form pkg:regex
     # the regex is used to ignore names within pkg. (default "fmt:.*").
     # see https://github.com/kisielk/errcheck#the-deprecated-method for details
-    ignore: fmt:.*,io/ioutil:^Read.*,io:Close
+    exclude-functions: fmt:.*,io/ioutil:^Read.*,io:Close
 
     # path to a file containing a list of functions to exclude from checking
     # see https://github.com/kisielk/errcheck#excluding-functions for details