Reduce change-version.js's scope of modifications

This PR is a suggestion to reduce the build/change-version.js scope of modifications after having seen those issues while releasing Boosted:

• Only modify Bootstrap version in package*.json
• Don't modify versions in site/content/docs files

Let's go back a few days with the commit before "Release v5.1.1":

git checkout b6855ae13817f516f63bd2916ead5243e07f210b
npm i
npm run release-version 5.1.0 5.1.1

From here:

• package.json and package-lock.json are modified but "bootstrap" package version isn't the only one to be modified:

─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
modified: package.json
─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
@ package.json:4 @
{
  "name": "bootstrap",
  "description": "The most popular front-end framework for developing responsive, mobile first projects on the web.",
  "version": "5.1.0",
  "version": "5.1.1",
  "config": {
    "version_short": "5.1"
  },
@ package.json:148 @
    "shelljs": "^0.8.4",
    "stylelint": "^13.13.1",
    "stylelint-config-twbs-bootstrap": "^2.2.3",
    "terser": "5.1.0",
    "terser": "5.1.1",
    "vnu-jar": "21.9.2"
  },
  "files": [

so if the release manager doesn't see it, some packages will be updated

• site/content/docs/5.1/**/*.md are modified (4 files are concerned when bumping from 5.1.0 to 5.1.1). This is the type of modifications that we can observe:

─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
modified: site/content/docs/5.1/customize/color.md
─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
@ color.md:112 @ Here's how you can use these in your Sass:

## Generating utilities

<small class="d-inline-flex px-2 py-1 font-monospace text-muted border rounded-3">Added in v5.1.0</small>
<small class="d-inline-flex px-2 py-1 font-monospace text-muted border rounded-3">Added in v5.1.1</small>

Bootstrap doesn't include `color` and `background-color` utilities for every color variable, but you can generate these yourself with our [utility API]({{< docsref "/utilities/api" >}}) and our extended Sass maps added in v5.1.0.
Bootstrap doesn't include `color` and `background-color` utilities for every color variable, but you can generate these yourself with our [utility API]({{< docsref "/utilities/api" >}}) and our extended Sass maps added in v5.1.1.

1. To start, make sure you've imported our functions, variables, mixins, and utilities.
2. Use our `map-merge-multiple()` function to quickly merge multiple Sass maps together in a new map.

But as well, those are unwanted updates because those versions must remain the same. I don't know well the history of the project but this type of information ("added in v5.x.x") is relatively new.