access first element with first() rather than get(0)

Fixes the clippy lint

```
warning: accessing first element with `self.transports.get(0)`
   --> pbs-tape/src/lib.rs:283:9
    |
283 | /         self.transports
284 | |             .get(0)
    | |___________________^ help: try: `self.transports.first()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#get_first
    = note: `#[warn(clippy::get_first)]` on by default
```

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>

pbs-tape/src/lib.rs

@@ -281,7 +281,7 @@ impl MtxStatus {
         // (are there changers exposing more than one?)
         // defaults to 0 for changer that do not report transports
         self.transports
-            .get(0)
+            .first()
             .map(|t| t.element_address)
             .unwrap_or(0u16)
     }

pbs-tape/src/sg_pt_changer.rs

@@ -850,7 +850,7 @@ mod test {
             .map(|desc| build_storage_descriptor(desc, trailing))
             .collect();
 
-        let (desc_len, address) = if let Some(el) = descs.get(0) {
+        let (desc_len, address) = if let Some(el) = descs.first() {
             (el.len() as u16, descriptors[0].address)
         } else {
             (0u16, 0u16)

src/server/realm_sync_job.rs

@@ -170,7 +170,7 @@ impl LdapRealmSyncJob {
                             "userid attribute `{user_id_attribute}` not in LDAP search result"
                         )
                     })?
-                    .get(0)
+                    .first()
                     .context("userid attribute array is empty")?
                     .clone();
 
@@ -233,7 +233,7 @@ impl LdapRealmSyncJob {
         existing_user: Option<&User>,
     ) -> User {
         let lookup = |attribute: &str, ldap_attribute: Option<&String>, schema: &'static Schema| {
-            let value = result.attributes.get(ldap_attribute?)?.get(0)?;
+            let value = result.attributes.get(ldap_attribute?)?.first()?;
             let schema = schema.unwrap_string_schema();
 
             if let Err(e) = schema.check_constraints(value) {