Update tested Go versions (and deps)

[benjaminjkraft]

Go 1.18 is out! So we should run tests on it.

Additionally, gqlgen had some issues with it (see 99designs/gqlgen#1961
and golang/go#45584) so I updated that too, which updated some other
things.

Finally, latest gqlgen requires 1.16+, and it's time for us to do the
same anyway, so we can use embed and other newer goodies. So I
dropped running tests for 1.14 and 1.15, and bumped the module language
version.

Test plan: make check

I have:

• Written a clear PR title and description (above)
• Signed the Khan Academy CLA
• Added tests covering my changes, if applicable
• Included a link to the issue fixed, if applicable
• Included documentation, for new features
• Added an entry to the changelog

[csilvers]

Are you sure you've signed the khan academy CLA? :-P

Change looks good to me. You may want to add the special .gitattributes thing that tells github to hide diffs for go.sum.

[benjaminjkraft]

Are you sure you've signed the khan academy CLA? :-P

I did! I realized while writing the checklist that now I actually ought to!

Change looks good to me. You may want to add the special .gitattributes thing that tells github to hide diffs for go.sum.

Ah, good idea. I marked it generated so you can still expand the diff.

Also note I added to this removal of 1.14 and 1.15 (PR message now updated).

[csilvers]

Also note I added to this removal of 1.14 and 1.15 (PR message now updated).

Yeah, I assume that's a gqlgen limitation?

[benjaminjkraft]

Yeah. Makes sense for us too, so we can use embed (PR to follow).