chore: update go to 1.18

[rainest]

Update to Go 1.18 and tidy mod.

If we're lucky, this will fix the golangci-lint errors we hit with the latest version.

Edit: it did, after some further changes to the action version, but also added a bunch of new formatting requirements, so an additional commit to run everything through gofumpt.

[codecov-commenter]

Codecov Report

Merging #164 (37a712b) into main (6a67fee) will decrease coverage by 1.08%.
The diff coverage is 91.27%.

@@            Coverage Diff             @@
##             main     #164      +/-   ##
==========================================
- Coverage   54.14%   53.06%   -1.09%     
==========================================
  Files          44       44              
  Lines        3991     3884     -107     
==========================================
- Hits         2161     2061     -100     
+ Misses       1374     1367       -7     
  Partials      456      456              

Flag	Coverage Δ
2.0.5	39.26% <58.72%> (-0.26%)	⬇️
2.1.4	39.10% <58.13%> (-0.23%)	⬇️
2.2.2	39.10% <58.13%> (-0.23%)	⬇️
2.3.3	39.10% <58.13%> (-0.23%)	⬇️
2.4.0	39.10% <58.13%> (-0.23%)	⬇️
2.5.1	39.10% <58.13%> (-0.23%)	⬇️
2.6.0	39.10% <58.13%> (-0.23%)	⬇️
2.7.0	39.10% <58.13%> (-0.23%)	⬇️
2.8.0	39.10% <58.13%> (-0.23%)	⬇️
community	39.26% <58.72%> (-0.26%)	⬇️
enterprise	51.67% <90.11%> (-1.13%)	⬇️
enterprise-1.5.0.11	51.67% <90.11%> (-1.13%)	⬇️
enterprise-2.1.4.6	50.66% <87.79%> (-1.05%)	⬇️
enterprise-2.2.1.3	50.66% <87.79%> (-1.05%)	⬇️
enterprise-2.3.3.4	50.66% <87.79%> (-1.05%)	⬇️
enterprise-2.4.1.3	50.66% <87.79%> (-1.05%)	⬇️
enterprise-2.5.1.2	50.66% <87.79%> (-1.05%)	⬇️
enterprise-2.6.0.2	50.66% <87.79%> (-1.05%)	⬇️
enterprise-2.7.0.0	50.66% <87.79%> (-1.05%)	⬇️
enterprise-2.8.0.0	50.66% <87.79%> (-1.05%)	⬇️
integration	53.06% <91.27%> (-1.09%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
kong/custom_entity_service.go	0.00% <0.00%> (ø)
kong/utils.go	77.36% <0.00%> (ø)
kong/admin_service.go	22.89% <61.53%> (-2.00%)	⬇️
kong/rbac_user_service.go	34.88% <77.77%> (-2.14%)	⬇️
kong/endpoint_permission_service.go	26.41% <80.00%> (-2.42%)	⬇️
kong/acl_service.go	62.50% <100.00%> (-1.24%)	⬇️
kong/basic_auth_service.go	62.50% <100.00%> (-1.24%)	⬇️
kong/ca_certificate_service.go	55.68% <100.00%> (-1.93%)	⬇️
kong/certificate_service.go	55.68% <100.00%> (-1.93%)	⬇️
kong/client.go	60.19% <100.00%> (ø)
... and 21 more

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