fix(MessageBar): Replace send button icon

[gabipodolnikova]

Replace the PaperPlaneIcon used by SendButton with the RH Ui Send icon from @patternfly/react-icons.

This currently uses the prerelease dependency @patternfly/react-icons@6.6.1-prerelease.2, released after PF-4576. The dependency can be updated to the stable release without code changes once available.

Restore PatternFly's default plain icon-button interaction styles while preserving the brand color for the default state.

Fixes #880

Tests:

• npx jest --runInBand packages/module/src/MessageBar/SendButton.test.tsx
• npm run build -w @patternfly/chatbot
• npx eslint packages/module/src/MessageBar/SendButton.tsx

[patternfly-build]

Preview: https://chatbot-pr-chatbot-903.surge.sh

A11y report: https://chatbot-pr-chatbot-903-a11y.surge.sh

[rebeccaalpert]

This is 100% my fault - would you be willing to use the filled version (if it exists?): https://www.figma.com/design/MSr6kVEOuAxmPOkjg7x8PO/PatternFly-6%3A-Patterns-%26-Extensions?m=auto&node-id=5190-4959&t=kak1QKBUdZzkndwZ-1

Looking at Kayla's design, we want Rh UI Send Fill Icon instead. Everything else looks great to me. 100% my fault.

[kaylachumley]

Hi! Thanks for working on this. +1 on the comment about the rh filled send icon change.

Does this pr also include other icons in the message bar being reverted back to the standard icon button styling? If so, it looks like the attach plus icon needs to be changed from subtle icon color to regular icon color and same for the other variation of attach(paper clip icon). Thanks!