Update to go 1.18 and use some generics

[rowanseymour]

It's already on its first minor release so feels safe to start dipping our toes in here...

[codecov]

Codecov Report

Merging #625 (0eddbfd) into main (f06fb72) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #625      +/-   ##
==========================================
- Coverage   61.99%   61.93%   -0.06%     
==========================================
  Files         130      130              
  Lines       12859    12839      -20     
==========================================
- Hits         7972     7952      -20     
  Misses       4031     4031              
  Partials      856      856              

Impacted Files	Coverage Δ
core/models/contacts.go	63.66% <100.00%> (ø)
core/models/sessions.go	54.86% <100.00%> (ø)
core/models/utils.go	80.43% <100.00%> (-5.93%)	⬇️

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