secretservice: use the correct MIME content type (#2475)

Secret Service backends that validate MIME content types, such as
oo7-daemon on Fedora 45, reject credential writes with 'Invalid content
type: plain/text'. Authentication succeeds, but the credential is never
stored and subsequent Git operations prompt again.

Use the standard text/plain content type for secret values so these
backends can store credentials.

Existing credentials remain readable because lookups use the service and
account attributes, and GCM reads the secret bytes without checking the
content type. Only new or updated secrets get the corrected content
type; no migration of existing entries is needed.

Fixes #2473

Author: mjcheetham

src/Core/Interop/Linux/SecretServiceCollection.cs

@@ -16,7 +16,7 @@ public class SecretServiceCollection : ICredentialStore
         private const string SchemaName = "com.microsoft.GitCredentialManager";
         private const string ServiceAttributeName = "service";
         private const string AccountAttributeName = "account";
-        private const string PlainTextContentType = "plain/text";
+        private const string PlainTextContentType = "text/plain";
 
         private readonly string _namespace;