fix: temporary fix for validation icon positioning (Closes #2599)

[tmorehouse]

Description of Pull Request:

Temporary fix for cssnano bug: cssnano/cssnano#712

"Fixed" by moving center to last value in background-position property

Bug will/may affect Nuxt.JS users and BootstrapVue docs site

Closes #2599

PR checklist:

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Enhancement to an existing feature
• ARIA accessibility
• Documentation update
• Other, please describe: SCSS + cssnano issue

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact:

The PR fulfills these requirements:

• It's submitted to the dev branch, not the master branch
• When resolving a specific issue, it's referenced in the PR's title (i.e.
  fixes #xxxx[,#xxxx], where "xxxx" is the issue number)
• The PR should address only one issue or feature. If adding multiple features or fixing a bug
  and adding a new feature, break them into separate PRs if at all possible.
• PR titles should following the
  Conventional Commits naming convention (i.e.
  "fix(alert): not alerting during SSR render", "docs(badge): Updated pill examples, fix typos",
  "chore: fix typo in docs", etc). This is very important, as the CHANGELOG is generated
  from these messages.

If new features/enhancement/fixes are added or changed:

• Includes documentation updates (including updating the component's package.json for slot and
  event changes)
• New/updated tests are included and passing (if required)
• Existing test suites are passing
• The changes have not impacted the functionality of other components or directives
• ARIA Accessibility has been taken into consideration (does it affect screen reader users or
  keyboard only users? clickable items should be in the tab index, etc)

If adding a new feature, or changing the functionality of an existing feature, the PR's
description above includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a
  suggestion issue first and wait for approval before working on it)

[codecov]

Codecov Report

Merging #2607 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #2607   +/-   ##
=======================================
  Coverage   70.47%   70.47%           
=======================================
  Files         172      172           
  Lines        3170     3170           
  Branches      923      923           
=======================================
  Hits         2234     2234           
  Misses        680      680           
  Partials      256      256

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[tmorehouse]

On hold to see when cssnano fixes the issue, and when Nuxt.JS updates dependencies to use new cssnano.

[tmorehouse]

It appears that it may be a while before cssnano is fixed: cssnano/cssnano#712 (comment)

[jacobmllr95]

@tmorehouse Hmm, then let get this in. It isn't that much css and we can remove it anytime again.