validation: Skip VerifyDB checks of level >=3 if dbcache is too small

Summary:
The previous behavior, skipping some L3 DisconnectBlock calls,
but still attempting to reconnect these blocks at L4, makes
ConnectBlock assert.

The variable skipped_l3_checks is introduced because even with an
insufficient cache for the L3 checks, the L1/L2 checks in the same
loop should still be completed.

This is a partial backport of core#27009
https://github.com/bitcoin/bitcoin/pull/27009/commits/61431e3a57b5613d8715c93c6eae0058e0217eaa

Fixes core#25563.

Test Plan:
src/bitcoind -dbcache=10

src/bitcoin-cli verifychain 4 10000

This produces the following output in the bitcoind terminal:

2023-12-06T12:57:54Z Verifying last 10000 blocks at level 4
2023-12-06T12:57:54Z [0%]...[10%]...[20%]...[30%]...[40%]...[50%]...Skipped verification of level >=3 (insufficient database cache size). Consider increasing -dbcache.
2023-12-06T12:57:57Z [DONE].
2023-12-06T12:57:57Z No coin database inconsistencies in last 10000 blocks (9992 transactions)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14921