Improve startup checks for insecure notary configs (#5392)

It's not really a problem to trust notary responses signed by the old key so
long as we are also doing TLS validation.

This commit adds a check to the config parsing code at startup to check that
we do not have the insecure matrix.org key without tls validation, and refuses
to start without it.

This allows us to remove the rather alarming-looking warning which happens at
runtime.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/5392.bugfix b/changelog.d/5392.bugfix
new file mode 100644
index 0000000000..295a7cfce1
--- /dev/null
+++ b/changelog.d/5392.bugfix
@@ -0,0 +1 @@
+Remove redundant warning about key server response validation.