subscriber: update minimum tracing-core dep to 0.1.28 (#2190)

## Motivation

PR #2008 added a new method to the `tracing_core::Subscriber` trait, and
modified `tracing-subscriber` to and implement that method. However,
that PR did *not* increase the minimum `tracing-core` dependency for the
`tracing-subscriber` crate.

This means that if a dependent of `tracing-subscriber` updates *only*
`tracing-subscriber` dependency version (e.g. by running
`cargo update -p tracing-subscriber`), it will not also update its
`tracing-core` version to one that contains the new method, and
`tracing-subscriber` will fail to compile. This is a common occurrence
with projects using Dependabot, for example.

## Solution

This commit updates `tracing-subscriber`'s minimum `tracing-core` dep to
0.1.28. Once this merges, I'll release 0.3.13 of `tracing-subscriber`
and yank 0.3.12.

Author: hawkw

tracing-subscriber/Cargo.toml

@@ -38,7 +38,7 @@ valuable = ["tracing-core/valuable", "valuable_crate", "valuable-serde", "tracin
 local-time = ["time/local-offset"]
 
 [dependencies]
-tracing-core = { path = "../tracing-core", version = "0.1.22", default-features = false }
+tracing-core = { path = "../tracing-core", version = "0.1.28", default-features = false }
 
 # only required by the filter feature
 tracing = { optional = true, path = "../tracing", version = "0.1", default-features = false }