Fix blob's gossip subscriptions

[tbenr]

We are incorrectly calculating relevant blobs topics for upcoming forks.

When new specConfig has been introduced, an assumption has been broken in Eth2GossipTopicFilter, which was passing the current milestone to getAllTopics even when calculating topics for the future forks.
It was fine because getAllTopics is only getting the config and from the spec, which in previous specConfig design was always the "latest defined milestone config".
With recent changes, it is no more the case, so we need to pick config from the correct milestone.

Unit test was broken because it was passing the wrong milestone (the next fork milestone, instead of the current one)

I fixed the bug and the tests.

Moreover the Electra blobs bump would have break getAllTopics in blob topics calculation. Thanks to new dynamic spec config, that works transparently now.

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

[rolfyone]

LGTM