bluetooth: host: Deprecated BT_CONN_TX_MAX

After https://github.com/zephyrproject-rtos/zephyr/pull/72090

,
`conn_tx_alloc` no longer blocks, and each buffer always has a
corresponding `bt_conn_tx` object. This eliminates the need to configure
the number of `bt_conn_tx` objects via `CONFIG_BT_CONN_TX_MAX`, since
every buffer now carries its own context even when no callback is used.

This commit deprecates `CONFIG_BT_CONN_TX_MAX` as it is no longer
necessary. Instead, `CONFIG_BT_BUF_ACL_TX_COUNT` is used to allocate
`bt_conn_tx` objects for outgoing ACL data. ZLL already uses
`CONFIG_BT_BUF_ACL_TX_COUNT` to configure the number of outgoing ACL
packets. With this change, modifying the packet count will automatically
adjust the number of corresponding contexts, preventing both context
starvatoin and underutilization.

This approach also aligns with ISO, where the number of `bt_conn_tx`
objects for outgoing ISOdata matches `CONFIG_BT_ISO_TX_BUF_COUNT`.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>