Snowbridge Beacon header age check (#3727)

## Bug Explanation
Adds a check that prevents finalized headers with a gap larger than the
sync committee period being imported, which could cause execution
headers in the gap being unprovable. The current version of the Ethereum
client checks that there is a header at least every sync committee, but
it doesn't check that the headers are within a sync period of each
other. For example:

Header 100 (sync committee period 1)
Header 9000 (sync committee period 2)
(8900 blocks apart)

These headers are in adjacent sync committees, but more than the sync
committee period (8192 blocks) apart.

The reason we need a header every 8192 slots at least, is the header is
used to prove messages within the last 8192 blocks. If we import header
9000, and we receive a message to be verified at header 200, the
`block_roots` field of header 9000 won't contain the header in order to
do the ancestry check.

## Environment
While running in Rococo, this edge case was discovered after the relayer
was offline for a few days. It is unlikely, but not impossible, to
happen again and so it should be backported to polkadot-sdk 1.7.0 (so
that
[polkadot-fellows/runtimes](https://github.com/polkadot-fellows/runtimes)
can be updated with the fix).

Our Ethereum client has been operational on Rococo for the past few
months, and this been the only major issue discovered so far.

### Unrelated Change
An unrelated nit: Removes a left over file that should have been deleted
when the `parachain` directory was removed.

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Co-authored-by: claravanstaden <Cats 4 life!>