backend/s3: require custom endpoint for Lyve Cloud v2 support

Lyve Cloud v2 no longer provides a shared S3 endpoint like v1 did. Instead, each customer receives
a unique, reseller-specific endpoint. To reflect this change, the S3 backend now requires users to
manually enter their endpoint when selecting Lyve Cloud as a provider.
Previously, users selected from a list of hardcoded Lyve Cloud v1 endpoints. This was not compatible
with Lyve Cloud v2 accounts and could cause confusion or misconfiguration.

This change:
- Removes outdated pre-defined endpoint selection for Lyve Cloud
- Requires users to provide their own endpoint
- Adds a format example to guide correct usage

Before: Users selected a fixed endpoint from a list (v1 only)
After:  Users must input their own endpoint (v2-compatible)

README.md

@@ -95,6 +95,7 @@ Rclone *("rsync for cloud storage")* is a command-line program to sync files and
   * rsync.net [:page_facing_up:](https://rclone.org/sftp/#rsync-net)
   * Scaleway [:page_facing_up:](https://rclone.org/s3/#scaleway)
   * Seafile [:page_facing_up:](https://rclone.org/seafile/)
+  * Seagate Lyve Cloud ["page_facing_up:](https://rclone.org/s3/#lyve)
   * SeaweedFS [:page_facing_up:](https://rclone.org/s3/#seaweedfs)
   * Selectel Object Storage [:page_facing_up:](https://rclone.org/s3/#selectel)
   * SFTP [:page_facing_up:](https://rclone.org/sftp/)