Merge bitcoin/bitcoin#31096: Package validation: accept packages of size 1

32fc59796f rpc: Allow single transaction through submitpackage (glozow)

Pull request description:

  There's no particular reason to restrict single transaction submissions with submitpackage. This change relaxes the RPC checks as enables the `AcceptPackage` flow to accept packages of a single transaction.

  Resolves #31085

ACKs for top commit:
  naumenkogs:
    ACK https://github.com/bitcoin/bitcoin/commit/32fc59796f74a2941772b5ec2755b1319132cd9c
  achow101:
    ACK 32fc59796f
  glozow:
    ACK 32fc59796f

Tree-SHA512: ffed353bfdca610ffcfd53b40b76da05ffc26df6bac4b0421492e067bede930380e03399d2e2d1d17f0e88fb91cd8eb376e3aabebbabcc724590bf068d09807c

doc/policy/packages.md

@@ -74,7 +74,7 @@ The following rules are only enforced for packages to be submitted to the mempoo
 enforced for test accepts):
 
 * Packages must be child-with-unconfirmed-parents packages. This also means packages must contain at
-  least 2 transactions. (#22674)
+  least 1 transaction. (#31096)
 
    - *Rationale*: This allows for fee-bumping by CPFP. Allowing multiple parents makes it possible
      to fee-bump a batch of transactions. Restricting packages to a defined topology is easier to