Timestamp schema pattern should allow Z instead of zero offset (#3420)

`2024-11-29T12:32:11.444600549+01:00` is accepted,
`2024-11-29T11:32:11.444600549Z` is not. But it should, see [1], it's a
valid format. tmt ingests the values, but does not parse them, just
passes them around, so changing the pattern should be enough to accept
this format.

[1] fedora-java/javapackages-validator#176

Author: happz

tmt/schemas/common.yaml

@@ -502,4 +502,4 @@ definitions:
   timestamp:
     type: string
     # yamllint disable-line rule:line-length
-    pattern: "^\\d{2,}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}\\.\\d+\\+\\d{2}:\\d{2}$"
+    pattern: "^\\d{2,}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}\\.\\d+(?:(?:\\+\\d{2}:\\d{2})|Z)$"