Use default p2a anchor

[ekrembal]

Issue Description

With Bitcoin Core 28.0, a special anchor output type was introduced that can have a 240-sat dust output:

    TxOut {
        value: Amount::from_sat(240),
        script_pubkey: ScriptBuf::from_hex("51024e73").unwrap(),
    };

Now, with V3 transactions, we can set transaction fees to 0, add a 240-sat anchor output, and use TRUC.
Note that transactions should not exceed 40k bytes. See: 4. A TRUC transaction cannot have a sigop-adjusted virtual size larger than 10,000 vB. [12]

We should fit assert transactions to this limit and assert begin tx must not have outputs more than 10000/43=230.

• Get rid of min relay fee constants
• Update the anchor output constant by setting DUST_VALUE to 240; we can update this later to 0 with Bitcoin Core 29.0 (see: PR #30239)