Renaming default branch

[awvwgk]

See fortran-lang/fpm#421 for details.

Please collect references in @fortran-lang projects to files inside this repository such that we can fix those links after renaming.

❯ rg master . .github
.github/workflows/doc-deployment.yml
42:        if: github.ref != 'refs/heads/master' && ! startsWith( github.ref, 'refs/tags/' )
63:        if: github.event_name == 'push' && github.repository == 'fortran-lang/stdlib' && ( startsWith( github.ref, 'refs/tags/' ) || github.ref == 'refs/heads/master' )
69:          publish_branch: master

./CONTRIBUTING.md
12:[stdlib license](https://github.com/fortran-lang/stdlib/blob/master/LICENSE).
25:[Fortran stdlib style guide](https://github.com/fortran-lang/stdlib/blob/master/STYLE_GUIDE.md)
45:   Try to reproduce it with the latest version from the master branch.
77:[this document](https://github.com/fortran-lang/stdlib/blob/master/WORKFLOW.md)
94:  [Fortran stdlib style guide](https://github.com/fortran-lang/stdlib/blob/master/STYLE_GUIDE.md).

./STYLE_GUIDE.md
84:[`doc/specs/`](https://github.com/fortran-lang/stdlib/tree/master/doc/specs).

./API-doc-FORD-file.md
28:project_download: https://github.com/fortran-lang/stdlib/archive/master.zip
57:[FORD project file]: https://github.com/fortran-lang/stdlib/blob/master/API-doc-FORD-file.md