Bump fontbox from 2.0.20 to 2.0.21 (JabRef#6883)

Bumps fontbox from 2.0.20 to 2.0.21.

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

build.gradle

@@ -106,7 +106,7 @@ dependencies {
     implementation fileTree(dir: 'lib', includes: ['*.jar'])
 
     implementation 'org.apache.pdfbox:pdfbox:2.0.21'
-    implementation 'org.apache.pdfbox:fontbox:2.0.20'
+    implementation 'org.apache.pdfbox:fontbox:2.0.21'
     implementation 'org.apache.pdfbox:xmpbox:2.0.20'
 
     implementation group: 'org.apache.commons', name: 'commons-csv', version: '1.8'