Update CI to run 1.18

[andrerfcsantos]

Related to #2129

Summary of the changes:

• Make actions refer Go 1.18
• Update golangci-lint of the test script to a version that accepts go.mod files referencing go versions < 1.18
• Copy strings.Title in place in exemplar solutions to bypass a linter check of deprecation (strings.Title is deprecated, and using the alternative is not viable)

[github-actions]

Dear andrerfcsantos

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  • Improvements to the instructions.md file should be made in the problem-spec repo so that all tracks can benefit.
    You can open a PR there instead.
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