Update a few CI steps to node 18

We upgraded docs to node 18 in #2066, way back in April 2023. However,
the nvmrc and some CI steps were left running on node 16.

That hasn't been a problem until today, when I tried to update
netlify-cli to a version that requires node 18 at a minimum. I thought
I'd sneak these changes into the netlify PR, since they're CI only and
don't impact prod.

alpine 3.16 isn't the latest alpine, but it's the one that has node 18
rather than node 20.

.buildkite/pipeline.yml

@@ -59,7 +59,7 @@ steps:
               login: true
               account-ids: public.ecr.aws
           - docker#v3.7.0:
-              image: "public.ecr.aws/docker/library/node:lts-alpine3.14"
+              image: "public.ecr.aws/docker/library/node:lts-alpine3.16"
 
       - label: ":lint-roller: Linting text and markdown"
         agents:
@@ -83,9 +83,7 @@ steps:
           # version, is way faster because we don't have to wait for Ruby gems to
           # install.
           - docker#v3.7.0:
-              # Alpine 3.14 - EOS 01 May 2023
-              # lts = Node 1415.1 - 2023-04-30
-              image: "public.ecr.aws/docker/library/node:lts-alpine3.14"
+              image: "public.ecr.aws/docker/library/node:lts-alpine3.16"
 
       - label: "Validate YAML"
         command:
@@ -100,7 +98,7 @@ steps:
               login: true
               account-ids: public.ecr.aws
           - docker#v3.7.0:
-              image: "public.ecr.aws/docker/library/node:lts-alpine3.14"
+              image: "public.ecr.aws/docker/library/node:lts-alpine3.16"
 
       - label: ":lint-roller: :markdown: Linting the Markdown"
         command: npm run -y mdlint
@@ -113,7 +111,7 @@ steps:
               login: true
               account-ids: public.ecr.aws
           - docker#v3.7.0:
-              image: "public.ecr.aws/docker/library/node:lts-alpine3.14"
+              image: "public.ecr.aws/docker/library/node:lts-alpine3.16"
 
       - label: ":snake: Linting markdown files for snake case"
         command: npx -y @ls-lint/[email protected]
@@ -126,7 +124,7 @@ steps:
               login: true
               account-ids: public.ecr.aws
           - docker#v3.7.0:
-              image: "public.ecr.aws/docker/library/node:lts-alpine3.14"
+              image: "public.ecr.aws/docker/library/node:lts-alpine3.16"
 
   - label: ":rspec: RSpec"
     depends_on: linting

.nvmrc

@@ -1 +1 @@
-16.19.1
+18.16.0