The plugin (or something else) must be pulling in an old version of Flying Saucer. Note that FS wasn't officially released on Maven until version 9. Various 3rd parties had released various versions with various group and artifact names. You'll have to manually exclude whatever is being pulled in.
I did double-check that the released version of 9.0.2 uses the proper signature for iText 2.1.x (see below).
private void setMetricDefaults();
Signature: ()V
Code:
0: aload_0
1: ldc #12; //float -50.0f
3: putfield #8; //Field _underlinePosition:F
6: aload_0
7: ldc #13; //float 50.0f
9: putfield #9; //Field _underlineThickness:F
12: aload_0
13: getfield #4; //Field _font:Lcom/lowagie/text/pdf/BaseFont;
16: bipush 120
18: invokevirtual #14; //Method com/lowagie/text/pdf/BaseFont.getCharBBox:(I)[I