Update FreeBSD versions in Cirrus CI. (#355)

Drop freebsd-12-4 which seems to be unavailable now, switch from
freebsd-14-0-snap to freebsd-14-0, and add freebsd-15-0-snap.

Author: sunfishcode

.cirrus.yml

@@ -2,9 +2,9 @@
 # at revision 7f4774e76bd5cb9ccb7140d71ef9be9c16009cdf.
 
 task:
-  name: stable x86_64-unknown-freebsd-14-snap
+  name: stable x86_64-unknown-freebsd-15-snap
   freebsd_instance:
-    image_family: freebsd-14-0-snap
+    image_family: freebsd-15-0-snap
   setup_script:
     - curl https://sh.rustup.rs -sSf --output rustup.sh
     - sh rustup.sh --default-toolchain stable -y --profile=minimal
@@ -15,9 +15,9 @@ task:
     - cargo test --features=fs_utf8 --workspace
 
 task:
-  name: stable x86_64-unknown-freebsd-13
+  name: stable x86_64-unknown-freebsd-14
   freebsd_instance:
-    image_family: freebsd-13-3
+    image_family: freebsd-14-0
   setup_script:
     - curl https://sh.rustup.rs -sSf --output rustup.sh
     - sh rustup.sh --default-toolchain stable -y --profile=minimal
@@ -28,9 +28,9 @@ task:
     - cargo test --features=fs_utf8 --workspace
 
 task:
-  name: stable x86_64-unknown-freebsd-12
+  name: stable x86_64-unknown-freebsd-13
   freebsd_instance:
-    image_family: freebsd-12-4
+    image_family: freebsd-13-3
   setup_script:
     - curl https://sh.rustup.rs -sSf --output rustup.sh
     - sh rustup.sh --default-toolchain stable -y --profile=minimal