Bump gpgme gem version from 2.0.13 to 2.0.18

What does this MR do?

Requires version ~> 2.0.18 for the gpgme gem. Gemfile.lock was causing 2.0.13 to be installed. gpgme 2.0.13 has a bug in its configure script where the qttest library is incorrectly detected: If qttest is not installed, but other qt libraries are installed, gpgme configure assumes that qttest exists, leading to a compiler error.

What are the relevant issue numbers?

None, but should I have created an issue? On non-KDE systems with qt libraries installed (except qttest), gitlab installation fails when gpgme (version 2.0.13) is compiled. This older gem version falsely detects that qttest is installed (based on the presence of other qt libraries), and the compiler throws an error unable to find the qttest header. Version 2.0.16 (and possibly earlier) fix this configuration error by correctly detecting the absence of qttest.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer