fix(b-table): header cell overflow for .sr-only sort label (closes #6326)

[jacobmllr95]

Describe the PR

This PR positions the <b-table> header cell (<b-th>) relative when it contains a .sr-only sort label to work around overflow issues.

Closes #6326.

PR checklist

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

• Bugfix (fixes a boo-boo in the code) - fix(...), requires a patch version update
• Feature (adds a new feature to BootstrapVue) - feat(...), requires a minor version update
• Enhancement (augments an existing feature) - feat(...), requires a minor version update
• ARIA accessibility (fixes or improves ARIA accessibility) - fix(...), requires a patch or minor version update
• Documentation update (improves documentation or typo fixes) - chore(docs), requires a patch version update
• Other (please describe)

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

• No
• Yes (please describe since breaking changes require a minor version update)

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[codecov]

Codecov Report

Merging #6371 (d37e0a5) into dev (7d72605) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev     #6371   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          299       299           
  Lines        10107     10108    +1     
  Branches      2510      2510           
=========================================
+ Hits         10107     10108    +1     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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Impacted Files	Coverage Δ
src/components/table/helpers/mixin-thead.js	100.00% <ø> (ø)
src/components/table/helpers/mixin-sorting.js	100.00% <100.00%> (ø)

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[driehle]

@jacobmllr95 It looks like the change adding the position-relative class breaks the sticky header feature, see #6961.