Deprecate save-always input (#1452)

The `save-always` input added in v4 is not
working as intended due to
`post-if` expressions not supporting the input
context.
To avoid breaking users who have already added
this input to their workflows, it is being
deprecated now and will be removed
in the next major version (v5).
See https://github.com/actions/cache/issues/1315 for more details.

README.md

@@ -17,7 +17,6 @@ See ["Caching dependencies to speed up workflows"](https://docs.github.com/en/ac
 ### v4
 
 * Updated to node 20
-* Added a `save-always` flag to save the cache even if a prior step fails
 
 ### v3