CTA Block: Excessive bottom padding

While building https://github.com/canonical/canonical.com/pull/1406, I noticed a visual quirk with the [CTA block](https://vanillaframework.io/docs/patterns/cta/block): we have given it `padding-bottom: $spv--x-large` here:

https://github.com/canonical/vanilla-framework/blob/78211ca79d33f316815a660be4ff91089fef40d3/scss/_patterns_cta.scss#L19

This causes some extra spacing beneath the CTA block that is not present in CTA blocks that don't use our `.p-cta-block`.

Default:

Expected (no bottom padding):

Start in scss/_patterns_cta.scss at the padding declaration linked in the issue, then compare the .p-cta-block styling with CTA blocks that do not use that class. Verify the change against the supplied screenshots or the CTA block documentation; done means the extra bottom spacing is no longer present.

Written by the indexing model from the issue text.