update ts; remove suppressImplicitAnyIndexErrors

[ChristopherChudzicki]

What are the relevant tickets?

An update on the way to #65

Description (What does it do?)

This PR updates Typescript and removes suppressImplicitAnyIndexErrors. The suppressImplicitAnyIndexErrors setting is deprecated and will be removed in TS 5.5, which probably isn't too far away. (5.3 is current).

Removing suppressImplicitAnyIndexErrors raised 9 errors that I suppressed with some typecasting.

How can this be tested?

Tests should pass.

It should be very clear from the changes that there are no functional changes in the source code. (Though it is possible the compiled code has changed due to TS update.) You can test the functionallity using mitodl/ocw-hugo-themes#1313 modified via

yarn remove @mitodl/course-search-utils
yarn add @mitodl/course-search-utils@https://github.com/mitodl/course-search-utils.git#c99bef7105cb4dc67503b39c29c6cc7ac0613a16
yarn start www

and check the search page.

[ChristopherChudzicki]

The updated linting packages were throwing errors with old node version; Node 15 is long unsupported, anyway.

[abeglova]

lgtm