[skip ci] Add 7.1 release notes for SQLite3 adapter improvements

[fractaledmind]

Motivation / Background

I have been pushing to get the SQLite3 adapter in a stronger place in advance of the 7.1 release. All in all, I have opened 3 pull requests to bring larger, existing ActiveRecord features to the SQLite3Adapter:

• support auto-populating columns and custom primary keys
• support generated columns
• support deferred foreign keys

In addition, I have also opened 3 additional pull requests to add some new, SQLite-specific features:

• support || concatenation in default functions
• performance tune default connection configurations
• add retries option as alternative to timeout

As of today (26-09-2023), 4 are merged (#49290, #49287, #49349, #49352) and 2 are still open (#49346, #49376)

I am presuming, however, the the last open 2 will be merged shortly (reviews are done and good to go).

Detail

This Pull Request adds a summary of the notable changes to the SQLite3 adapter to the release notes

[skipkayhil]

Can you remove skip ci from the commit message? (Putting it in the PR title but not the commit message will run Github Actions and not Buildkite, which is what we want)

The text looks good but I'll wait for the other PRs to be merged 👍

[fractaledmind]

@skipkayhil Done

[yahonda]

generated columns #49346 and defrered foregn keys #49376 are open and they are not merged to main (yet), so at least the 7.1 release note should not include them.

[rafaelfranca]

Fixed in 8d2c455