[BACKPORT 2.18.4][PLAT-11216] Backup script failing on CentOS 7

Summary:
Original commit: 29eb6b1 / D29944
Restore was failing on prometheus directories because centos was taking single quotes as a literal. This diff removes the quotes so it treats prom_snapshot as a variable.

This was a regression from https://phorge.dev.yugabyte.com/D29466 which is in 2.18.4 and 2.20.0

Test Plan: Backup restore on ubuntu and centos 7 wih change, ensure prometheus dir are moved properly.

Reviewers: dshubin, sanketh

Reviewed By: dshubin

Subscribers: yugaware

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D29969

managed/devops/bin/yb_platform_backup.sh

@@ -635,8 +635,7 @@ restore_backup() {
     modify_service prometheus stop
     # Find snapshot directory in backup
     run_sudo_cmd "rm -rf ${PROMETHEUS_DATA_DIR}/*"
-    # snapshot_dir="${destination}/'${prom_snapshot:2}'*"
-    run_sudo_cmd "mv ${destination}/'${prom_snapshot:2}'* ${PROMETHEUS_DATA_DIR}"
+    run_sudo_cmd "mv ${destination}/${prom_snapshot:2}* ${PROMETHEUS_DATA_DIR}"
     if [[ "${yba_installer}" = true ]] && [[ "${migration}" = true ]]; then
       backup_targets=$(find "${yugabackup}" -name swamper_targets -type d)
       if  [[ "$backup_targets" != "" ]] && [[ -d "$backup_targets" ]]; then