fix(split): Use correct Kconfig for bumping TX buffers for discovery

[petejohanson]

Newer Zephyr uses a different Kconfig value to control buffers for GATT client discovery, so adjust our defaults to bump the correct value

Fixes: #3156

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.