graphviz errors in scaladocs for 2.12

graphviz produces errors during the generation of embedded svgs in the 2.12 scaladocs.

you can reproduce this with e.g. `sbt ++2.12 persistence-tck/doc`.

diagnosing by adding `-diagrams-debug` to the scaladoc options in `project/Doc.scala`, we get some more diagnostics:

```
model contains 36 documentable templates

**********************************************************************
The graphviz dot diagram tool has malfunctioned and will be restarted.

The following is the log of the failure:
DOT : malloc_consolidate(): unaligned fastbin chunk detected

Cleanup: Last template: org.apache.pekko.persistence.journal.JournalPerfSpec
Cleanup: Last dot input:
digraph G {
compound="true";
rankdir="TB";
node [margin="0.08,0.01", fontsize="8.00", style="filled,rounded", height="0.0", penwidth="1", shape="rect", width="0.0", fontname="Source Code Pro"];
edge [arrowsize="0.7", fontcolor="#aaaaaa", fontsize="9.00", color="#d4d4d4", fontname="Source Code Pro"];
subgraph clusterAll {
style="invis"
subgraph clusterOutgoing {
style="invis"
node6 [label=<scalatest.matchers.should.Matchers.Equalizer[JournalPerfSpec] >,fontcolor="#7d7d7d", color="#ababab", id="implicit-outgoing class|graph1_6", fillcolor="#e1e1e1", tooltip="org.apache.pekko.persistence.journal.JournalPerfSpec can be implicitly converted to scalatest.matchers.should.Matchers.Equalizer[JournalPerfSpec] by the implicit method convertToEqualizer in trait org.scalactic.TripleEquals"];
node5 [label=<scalatest.matchers.should.Matchers.AnyShouldWrapper[JournalPerfSpec] >,fontcolor="#7d7d7d", color="#ababab", id="implicit-outgoing class|graph1_5", fillcolor="#e1e1e1", tooltip="org.apache.pekko.persistence.journal.JournalPerfSpec can be implicitly converted to scalatest.matchers.should.Matchers.AnyShouldWrapper[JournalPerfSpec] by the implicit method convertToAnyShouldWrapper in trait org.scalatest.matchers.should.Matchers"];
node4 [label=<scalactic.TripleEquals.Equalizer[JournalPerfSpec] >,fontcolor="#7d7d7d", color="#ababab", id="implicit-outgoing class|graph1_4", fillcolor="#e1e1e1", tooltip="org.apache.pekko.persistence.journal.JournalPerfSpec can be implicitly converted to scalactic.TripleEquals.Equalizer[JournalPerfSpec] by the implicit method convertToEqualizer in trait org.scalactic.TripleEquals"];
node3 [label=<scalatest.Assertions.Equalizer[JournalPerfSpec] >,fontcolor="#7d7d7d", color="#ababab", id="implicit-outgoing class|graph1_3", fillcolor="#e1e1e1", tooltip="org.apache.pekko.persistence.journal.JournalPerfSpec can be implicitly converted to scalatest.Assertions.Equalizer[JournalPerfSpec] by the implicit method convertToEqualizer in trait org.scalactic.TripleEquals"];
node3 -> node4 -> node5 -> node6 [constraint="false", style="invis", minlen="0.0"];
}
subgraph clusterThis {
style="invis"
node0 [label=<JournalPerfSpec >,fontcolor="#ffffff", URL="#inheritance-diagram", color="#418565", id="this class|graph1_0", fillcolor="#418565", tooltip="org.apache.pekko.persistence.journal.JournalPerfSpec (this class)"];
}

node0 -> node3 [id="implicit-outgoing|0_3", tooltip="JournalPerfSpec can be implicitly converted to scalatest.Assertions.Equalizer[JournalPerfSpec], scalactic.TripleEquals.Equalizer[JournalPerfSpec], scalatest.matchers.should.Matchers.AnyShouldWrapper[JournalPerfSpec], scalatest.matchers.should.Matchers.Equalizer[JournalPerfSpec]", constraint="false", minlen="2", ltail="clusterThis", lhead="clusterOutgoing", label="implicitly"];
}
node1 [label=<JavaJournalPerfSpec >,fontcolor="#ffffff", URL="../japi/journal/JavaJournalPerfSpec.html#inheritance-diagram", color="#418565", id="class|graph1_1", fillcolor="#418565", tooltip="org.apache.pekko.persistence.japi.journal.JavaJournalPerfSpec"];
node2 [label=<JournalSpec >,fontcolor="#ffffff", URL="JournalSpec.html#inheritance-diagram", color="#418565", id="class|graph1_2", fillcolor="#418565", tooltip="org.apache.pekko.persistence.journal.JournalSpec"];
node2 -> node0 [id="inheritance|graph1_2_0", tooltip="JournalPerfSpec is a subtype of JournalSpec", dir="back", arrowtail="empty"];
node0 -> node1 [id="inheritance|graph1_0_1", tooltip="JavaJournalPerfSpec is a subtype of JournalPerfSpec", dir="back", arrowtail="empty"];
}

Cleanup: Dot path: dot
Cleanup: Dot exit code: 134
**********************************************************************
Graphviz will be restarted...

Diagram generation running time breakdown:

diagrams model filtering
========================
count: 72 items
total time: 0 ms
average time: 0 ms
maximum time: 0 ms

diagrams model generation
=========================
count: 72 items
total time: 10 ms
average time: 0 ms
maximum time: 2 ms

dot diagram generation
======================
count: 21 items
total time: 6 ms
average time: 0 ms
maximum time: 1 ms

dot process running
===================
count: 21 items
total time: 387 ms
average time: 18 ms
maximum time: 277 ms

svg processing
==============
count: 21 items
total time: 81 ms
average time: 3 ms
maximum time: 28 ms

Broken images: 94
Fixed images: 0

```

I'm using graphviz 9.0.0, and passing that graph to dot manually works fine. Also 2.13 docs appear to be fine.

Perhaps we should just disable scaladoc diagrams for 2.12?