Minor documentation and script cleanups (#6868)

* Remove mention of no longer used IRC network

Channels exist on matrix, and on libera.chat now.

* Correctly match only files with .png extension

The current search would match files such as 'createpng'.

* Fix comparison in script

The result was always false, due to comparing a literal string instead of a variable.

* Use correct license files from upstream

Correct license files obtained from:
https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt
https://www.gnu.org/licenses/gpl-3.0.txt
https://www.gnu.org/licenses/old-licenses/lgpl-2.1.txt
https://www.gnu.org/licenses/lgpl-3.0.txt

* Refresh several shell scripts

This fixes several shellcheck warnings, as well as makes the code more
robust and have consistent codestyle between all the files.

* Trim excess whitespace

docs/CMakeLists.txt

@@ -53,7 +53,7 @@ if(UNIX)
         VERBATIM)
     add_custom_target(manpages ALL DEPENDS keepassxc.1 keepassxc-cli.1)
 
-    install(FILES 
+    install(FILES
         ${OUT_DIR}/keepassxc.1
         ${OUT_DIR}/keepassxc-cli.1
         DESTINATION ${CMAKE_INSTALL_MANDIR}/man1/)