feat(table): remove deprecated slot names, introduce new slot names

[jacobmllr95]

Describe the PR

This PR removes the deprecations from the <b-table> and <b-table-simple> components.

Users should now be using slots cell[field] instead of field, head[field] instead of HEAD_field, and foot[field] instead of FOOT_field. Noe these new slot names are different than teh ones introduced in 2.0.0-rc.28.

The changes are needed due to the new Vue v-slot syntax

PR checklist

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

• Bugfix
• Feature
• Enhancement
• ARIA accessibility
• Documentation update
• Other (please describe)

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

• No
• Yes (please describe)

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 #xxx[,#xxx]), where "xxx" is the issue number)
• It 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.
• The title should follow the Conventional Commits naming convention (i.e. fix(alert): not alerting during SSR render, docs(badge): update pill examples, fix typos, chore: fix typo in README, 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)
• Includes any needed TypeScript declaration file updates
• 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 #3866 into dev will decrease coverage by <.01%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             dev   #3866      +/-   ##
========================================
- Coverage   99.3%   99.3%   -0.01%     
========================================
  Files        233     233              
  Lines       4484    4473      -11     
  Branches    1270    1266       -4     
========================================
- Hits        4453    4442      -11     
  Misses        25      25              
  Partials       6       6

Impacted Files	Coverage Δ
src/components/table/helpers/normalize-fields.js	100% <ø> (ø)	⬆️
src/components/table/helpers/mixin-items.js	100% <ø> (ø)	⬆️
src/components/table/helpers/mixin-thead.js	100% <100%> (ø)	⬆️
src/components/table/helpers/mixin-tbody-row.js	100% <100%> (ø)	⬆️
src/components/table/helpers/mixin-filtering.js	100% <100%> (ø)	⬆️

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[tmorehouse]

Created an upstream issue at vuejs/vue#10390 regarding new v-slot syntax not working with slot names that start with [