Update dependency bootstrap-vue to v2.21.0

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
bootstrap-vue (source)	2.20.1 -> 2.21.0

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Release Notes

bootstrap-vue/bootstrap-vue

v2.21.0

Compare Source

Released: 2020-12-14

Features v2.21.0

• b-form-group: add content-cols props and scoped default slot (closes
  #​6095,
  #​6118)
  (#​6178)
  (fab6dc5)
• b-form-tags: add no-tags-remove prop (closes
  #​6162)
  (#​6163)
  (92de1f9)
• b-sidebar: add header slot
  (#​6179)
  (341b7f0)
• refactor: code enhancements for easier Vue 3 migration (closes
  #​6124,
  #​6139)
  (#​6141)
  (5bf6733)
• icons: update Bootstrap Icons to v1.2.1
  (#​6194)
  (799e272)
• icons: update Bootstrap Icons to v1.2.0
  (#​6180)
  (00682e5)

Bug Fixes v2.21.0

• b-form-datepicker: valueAsDate prop handling
  (#​6159)
  (5cb8e0c)
• b-form-datepicker/b-form-timepicker: label styles when in button-only mode (closes
  #​6172)
  (#​6186)
  (e8842ba)
• b-tabs: cleanup rendering logic
  (#​6154)
  (8aeb9e9)
• table: default sort compare logic for date strings
  (#​6153)
  (3696a1f)
• table: use original value for fallback when number parsing fails in defaultSortCompare()
  (c375ce9)

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