fix(form-group): don't render aria-labelledby on group when label-for provided (fixes #2933)

[tmorehouse]

Description of Pull Request:

Only render the aria-labelledby attribute on the form-group when label-for is not provided.

Also removes the duplicated aria-describedby from the form-group when label-for is set (as it appears on the input when label-for provided)

Fixes #2933

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:

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 #2936 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #2936      +/-   ##
==========================================
+ Coverage   92.04%   92.05%   +<.01%     
==========================================
  Files         205      205              
  Lines        3243     3247       +4     
  Branches      959      959              
==========================================
+ Hits         2985     2989       +4     
  Misses        190      190              
  Partials       68       68

Impacted Files	Coverage Δ
src/components/form-group/form-group.js	99% <100%> (+0.04%)	⬆️

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