Update dependency @gitlab/ui to v56.3.0

```yaml:frontmatter

Fix rendering of Vulnerability Report table

Removing `responsive` is fine, since before the BSV upgrade,
the `responsive` prop has no effect when `stacked` is given. So dropping
`responsive` preserves the old rendering behaviour of the root element
being `<table>`.

The `position: sticky !important` fix is required to work around
https://github.com/bootstrap-vue/bootstrap-vue/pull/6371.

In general, the layout approach here is quite brittle. The stick header
isn't visible for certain screen sizes anyway, since the filter bar
height is a constant rather than measured.

```

Update snapshots: aria attributes on b-table

Reason: https://github.com/bootstrap-vue/bootstrap-vue/commit/3f5a3095500c706a75f0f0d6015b0b2777051e1f

---

Update dropdown-related tests

Reason: https://github.com/bootstrap-vue/bootstrap-vue/pull/6865

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Update pypi installation snapshot

Reason: https://github.com/bootstrap-vue/bootstrap-vue/pull/6178

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Update modal-related snapshot

defaults for title / ignore-enforce-focus-selector removed in https://github.com/bootstrap-vue/bootstrap-vue/commit/5bf6733595091cc204d3acc0641f8f0301bcbe9c#diff-698a07032aec8fd0476d3d4d11834f24dfa87f31dcb77e529eb9d01fd1d42326
header-tag / footer-tag added in https://github.com/bootstrap-vue/bootstrap-vue/commit/c6dd70a787cdc711b3ce539a65f6aac273749874

---

Update b-link / b-button related snapshots

Reason: https://github.com/bootstrap-vue/bootstrap-vue/commit/05db323e6549de781b36bf73515d3ca1bc220050

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Update dropdown snapshot

Reason: https://github.com/bootstrap-vue/bootstrap-vue/pull/6339

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Fix tests relying on role=alert in form feedback

Reason: https://github.com/bootstrap-vue/bootstrap-vue/pull/6346

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Fix provide: syntax

Reason: https://github.com/vuejs/vue/issues/12854

---

Fix Startup CSS

---

Remove unnecessary style override

Now that `.gl-table` is applied to the `<table>` element (even though
it's in a `<div>` wrapper due to `responsive`), the header gets styled
correctly without this override.