Switch git repo URL from git: to https:

...hoping that it would fix the issue where recent builds on flathub started to
fail with

> Fetching git repo git://gerrit.libreoffice.org/core, ref refs/tags/libreoffice-6.4.5.2
> FB: Running: git fetch -p --no-recurse-submodules --depth=1 -f origin '+refs/tags/libreoffice-6.4.5.2:refs/tags/libreoffice-6.4.5.2'
> fatal: the remote end hung up unexpectedly
> Failed to download sources: module libreoffice: Child process exited with code 128

(e.g., <https://flathub.org/builds/#/builders/10/builds/2703>)

Author: stbergmann

org.libreoffice.LibreOffice.json

@@ -52,7 +52,7 @@
             "sources": [
                 {
                     "type": "git",
-                    "url": "git://gerrit.libreoffice.org/core",
+                    "url": "https://gerrit.libreoffice.org/core",
                     "branch": "libreoffice-6.4.5.2",
                     "disable-fsckobjects": true
                 },