Add gitkeep file to ignored towncrier files

Release 24.7.0 made this necessary by scanning for invalid filenames
Reference: twisted/towncrier#622

Author: johannaengland

towncrier.toml

@@ -6,6 +6,7 @@ underlines = ["", "", ""]
 title_format = "## [v{version}] - {project_date}"
 issue_format = "[#{issue}](https://github.com/Uninett/Argus-frontend/issues/{issue})"
 wrap = true
+ignore = [".gitkeep"]
 
 [[tool.towncrier.type]]
 directory = "security"