[backport#19469] rpc: deprecate banscore field in getpeerinfo

Summary:
41d55d30579358c805036201664ad6a1c1d48681 doc: getpeerinfo banscore deprecation release note (Jon Atack)
dd54e3796e633cfdf6954af306afd26eadc25116 test: getpeerinfo banscore deprecation test (Jon Atack)
8c7647b3fbbab03ea84071cf3cd2d0d2bf8be255 rpc: deprecate banscore field in rpc getpeerinfo (Jon Atack)

Pull request description:

Per https://github.com/bitcoin/bitcoin/pull/19219#discussion_r443074487 and https://github.com/bitcoin/bitcoin/pull/19219#issuecomment-652699592, this PR deprecates returning the `banscore` field in the `getpeerinfo` RPC, updates the help, adds a test, and updates the release notes. Related to #19464.

Backport of Core PR19469

Test Plan:

ninja all check
test_runner rpc_getpeerinfo_banscore_deprecation

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8798