…uld not be auto-set on first input when modal appears (elastic#194696)

Closes: elastic#194965
Closes: elastic#194966

# Description

- [x] elastic#194965 <br /> Focus is
currently being set on the first input in the "Create group" modal.
Screen reader users will hear the input name, but not get the title of
the modal read aloud this way, and it could be confusing. We should be
letting the EuiModal set focus naturally on the modal or close button so
screen reader users hear the title as expected.

- [x] elastic#194966 <br /> Focus must
be returned properly when I cancel the "Create group" workflow in
Services > Create service group modal.

# Changes Made

1. Removed:

```diff
- inputRef.current?.focus(); // autofocus on initial render
```

2. Added `aria-labelledby={modalTitleId}` for `EuiModal`. See
https://eui.elastic.co/#/layout/modal.
3. Slightly updated `Name` and `Color` validation.

# Screen

https://github.com/user-attachments/assets/6636f2dc-b9b7-4d4d-8144-90249f8327e7
(cherry picked from commit d576365)