fix(input-group): Support form-control-plaintext as a sibling

[tmorehouse]

PR #25871 added support for .form-control-plaintext inside an input-group (addressing issue #25870), but it did not include handling the situation of multiple inputs in the same input group (where .form-control-plaintext is not the first input).

This PR adds in the missing line to handle this situation.

[tmorehouse]

Hmmm now that I look at this, I just realized that .form-control-plaintext inside an input group doesn't have a border (so maybe this PR is not needed). Should borders be added to it when it is inside an input-group for consistent look with other input-groups or should it be left borderless?

And also noticed that .form-control-plaintext is missing the left and right padding (to line it's contents up with other controls that may be in the same form) when inside an input-group.

[MartijnCuppens]

No need for this PR, you can use the readonly attribute for this as mdo mentioned #25870 (comment)

[tmorehouse]

@MartijnCuppens that is a good point re readonly. I wonder if that suggestion should be mentioned in the input group documentation?