Clean up old Dockerfiles and their references

We used to use many images to build the final docs version image, but that has changed the last months, where we consolidated all of them into https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/dockerfiles/single.Dockerfile.

The old images can still be used and accessed by the older branches (for example 14.10), should we need to build them in the future.

This MR deletes the Dockerfiles and their respective CI jobs, so that we don't accidentally update any of those and the older pipelines break (see !3291 (comment 1181410575) for an explanation).

If you search for any of them, there's no linking reference left (I'm using ag instead of grep):

❯ ag "builder|onbuild|bootstrap" .


Gemfile
16:  gem 'builder', '~> 3.2.4'

Gemfile.lock
24:    builder (3.2.4)
301:  builder (~> 3.2.4)

content/frontend/shared/global_imports.js
2:import 'bootstrap';

content/assets/stylesheets/stylesheet.scss
141:// Override bootstrap's alert color
250:  /* Override Bootstrap theme color */

yarn.lock
219:"@babel/helper-builder-binary-assignment-operator-visitor@^7.18.6":
221:  resolved "https://registry.yarnpkg.com/@babel/helper-builder-binary-assignment-operator-visitor/-/helper-builder-binary-assignment-operator-visitor-7.18.6.tgz#f14d640ed1ee9246fb33b8255f08353acfe70e6a"
784:    "@babel/helper-builder-binary-assignment-operator-visitor" "^7.18.6"
1189:    bootstrap-vue "2.20.1"
2215:bootstrap-vue@2.20.1:
2217:  resolved "https://registry.yarnpkg.com/bootstrap-vue/-/bootstrap-vue-2.20.1.tgz#1b6cd4368632c1a6dd4a5ed161242baa131c3cd5"
2221:    bootstrap ">=4.5.3 <5.0.0"
2226:"bootstrap@>=4.5.3 <5.0.0", bootstrap@^4.6.1:
2228:  resolved "https://registry.yarnpkg.com/bootstrap/-/bootstrap-4.6.2.tgz#8e0cd61611728a5bf65a3a2b8d6ff6c77d5d7479"

lib/helpers/admonition.rb
5:    BOOTSTRAP_MAPPING = {
24:        %(<div class="admonition admonition-non-dismissable alert alert-#{BOOTSTRAP_MAPPING[kind]}">) +

jest.config.js
16:    'node_modules/(?!(@gitlab/(ui|svgs)|bootstrap-vue|vue-instantsearch|instantsearch.js)/)',

dockerfiles/single.Dockerfile
10:# Adapted from https://github.com/docker/docker.github.io/blob/publish-tools/Dockerfile.builder
20:#- Start of builder stage -#
22:FROM ruby:2.7.6-alpine3.16 AS builder
97:#- End of builder build stage -#
101:# Copy the ending HTML files from the previous 'builder' stage and copy them
106:# Clean out any existing HTML files, and copy the HTML from the builder stage
109:COPY --from=builder /site /usr/share/nginx/html

package.json
51:    "bootstrap": "^4.6.1",