Cherry-picks for 2.10.23-RC.2

[neilalexander]

Includes the following:

• Some tweaks to the NRG test helpers
• https://github.com/nats-io/nats-server/pull/6055
• https://github.com/nats-io/nats-server/pull/6061
• https://github.com/nats-io/nats-server/pull/6065
• https://github.com/nats-io/nats-server/pull/6041 (but with math/rand instead of math/rand/v2 due to an older Go version in CI for 2.10.x)
• https://github.com/nats-io/nats-server/pull/6066
• https://github.com/nats-io/nats-server/pull/6067
• https://github.com/nats-io/nats-server/pull/6069
• https://github.com/nats-io/nats-server/pull/6075
• https://github.com/nats-io/nats-server/pull/6082
• https://github.com/nats-io/nats-server/pull/6087
• https://github.com/nats-io/nats-server/pull/6086
• https://github.com/nats-io/nats-server/pull/6088
• https://github.com/nats-io/nats-server/pull/6089
• https://github.com/nats-io/nats-server/pull/6092
• https://github.com/nats-io/nats-server/pull/6096
• https://github.com/nats-io/nats-server/pull/6098
• https://github.com/nats-io/nats-server/pull/6097
• https://github.com/nats-io/nats-server/pull/6105
• https://github.com/nats-io/nats-server/pull/6104
• https://github.com/nats-io/nats-server/pull/6106
• https://github.com/nats-io/nats-server/pull/6109
• https://github.com/nats-io/nats-server/pull/6111
• https://github.com/nats-io/nats-server/pull/6112

Signed-off-by: Neil Twigg neil@nats.io