Replace old GIFs with symbols

Following a brief discussion with @MarkEWaite (https://github.com/jenkinsci/groovy-postbuild-plugin/pull/81#issuecomment-2404998761) I propose to get rid of the *.gif images.

In order to not break current pipelines an on-the-fly replacement should be implemented. See the following table for my proposal:

name	old	new	origin
completed.gif			core `status-blue`
db_in.gif			ionicons `cloud-upload-outline`
db_out.gif			ionicons `cloud-download-outline`
delete.gif			core `trash`
error.gif			core `status-red`
folder.gif			core `folder`
green.gif			emoji `green_square`
info.gif			core `information-circle`
red.gif			emoji `red_square`
save.gif			ionicon `save-outline`
success.gif			core `status-blue`
text.gif			core `document-text`
warning.gif			core `status-yellow`
yellow.gif			emoji `yellow_square`

Some of the icons do not have proper replacements from the core (https://github.com/jenkinsci/jenkins/tree/master/war/src/main/resources/images/symbols) or the ionicons-api-plugin. That's why I added emoji-symbols-api as a dependency to find the missing pieces.

Any feedback is highly welcome!

Added tests to cover the replacement

[x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
[x] Ensure that the pull request title represents the desired changelog entry
[x] Please describe what you did
[ ] Link to relevant issues in GitHub or Jira
[x] Link to relevant pull requests, esp. upstream and downstream changes
[x] Ensure you have provided tests - that demonstrates feature works or fixes the issue

jenkinsci / badge-plugin