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

utils/keepassxc-snap-helper.sh

@@ -118,9 +118,9 @@ BROWSER=$(whiptail \
 clear
 
 exitstatus=$?
-if [ $exitstatus = 0 ]; then
+if [[ $exitstatus == 0 ]]; then
     # Configure settings for the chosen browser
-    case "$BROWSER" in
+    case $BROWSER in
         1) setupFirefox ;;
         2) setupChrome ;;
         3) setupChromium ;;
@@ -132,7 +132,7 @@ if [ $exitstatus = 0 ]; then
     # Install the JSON file
     cd ~
     mkdir -p "$INSTALL_DIR"
-    echo "$JSON_OUT" > ${INSTALL_DIR}/${INSTALL_FILE}
+    echo "$JSON_OUT" > "${INSTALL_DIR}/${INSTALL_FILE}"
 
     whiptail \
         --title "Installation Complete" \