chore: Add DEPRECATED.md to list all out deprecated items

Since some functionality is deprecated without a warning message, it is
helpful to have a central location to list all deprecations.

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

DEPRECATED.md

@@ -0,0 +1,18 @@
+# Deprecated Features
+
+The following functionality of Firecracker is deprecated, and will be removed in
+a future major Firecracker release, in accordance with our
+[release policy](docs/RELEASE_POLICY.md).
+
+- \[[#2763](https://github.com/firecracker-microvm/firecracker/pull/2763)\] The
+  `vsock_id` body field in `PUT` requests on `/vsock`
+- \[[#2980](https://github.com/firecracker-microvm/firecracker/pull/2980)\] The
+  `mem_file_path` body field in `PUT` requests on `/snapshot/load`
+- \[[#2973](https://github.com/firecracker-microvm/firecracker/pull/2973)\]
+  MicroVM Metadata Service v1 (MMDSv1)
+- \[[#4126](https://github.com/firecracker-microvm/firecracker/pull/4126)\]
+  Static CPU templates
+- \[[#4209](https://github.com/firecracker-microvm/firecracker/pull/4209)\] The
+  `rebase-snap` tool
+- \[[#4500](https://github.com/firecracker-microvm/firecracker/pull/4500)\] The
+  `--start-time-cpu-us` and `--start-time-us` CLI arguments