move from OpenCOBOL to GnuCOBOL

Reasoning: the OpenCOBOL project was renamed to GnuCOBOL long time ago and the formerly had its last release in 2009 while GnuCOBOL is actively maintained. I suggest to rename the syntax and related files, along with the shortcut.

I think it is also reasonable to update the reserved words, you may see them in the current CI built documentation under "Appendix B + Appendix E for the mnemonics + Appendix C for the intrinsic functions".