Changed regex from 'only-digit' to 'hex'

Summary:
Parsing hex-tags with lowercase and uppercase characters;
Writing hex-tags only with uppercase characters

Forgot to change FromStr-function

Merge branch 'T22064-Convert-DICOM-hex-tags'

Test Plan: Unit test contained; was modified corresponding to the changes made to the module itself

Reviewers: goch, O1 MITK Core, kislinsk, floca

Reviewed By: O1 MITK Core, kislinsk, floca

Subscribers: kalali

Differential Revision: https://phabricator.mitk.org/D17

(cherry picked from commit 67a03f7695e39aa72eaddf6e75d71dba50c0b5d6)