Merge bitcoin/bitcoin#23383: Update libsecp256k1 subtree to current master

314195c8be3bd7db0d5817c4fb3aa85c84363ce9 Remove unnecessary cast in CKey::SignSchnorr (Pieter Wuille)
a1f76cdb22e3278a48d63dd23c1fe3308daedd8c Remove --disable-openssl-tests for libsecp256k1 configure (Pieter Wuille)
86dbc4d075decb82fbba837aaa283cf0561897ad Squashed 'src/secp256k1/' changes from be8d9c262f..0559fc6e41 (Pieter Wuille)

Pull request description:

  The motivation for this bump is getting rid of a cast in `CKey::SignSchnorr`; the `aux_rand` argument isn't modified by the `secp256k1_schnorrsig_sign` function, but was marked as non-`const` anyway. This is fixed now (bitcoin-core/secp256k1#966), and the cast is removed in this PR.

  There are a few other relevant changes:
  * (bitcoin-core/secp256k1#956): replaces a runtime-computed table with a precomputed one; this adds arouns 1 MiB to the binary size, but is a step towards significantly simplifying the API. If 1 MiB is too much, it can be reduced by 2 or 4 (or more) for a slight verification performance reduction.
  * (bitcoin-core/secp256k1#983): removes (test/bench only) OpenSSL support entirely, removing the need to pass `--disable-openssl-tests` (see #23314).
  * (bitcoin-core/secp256k1#810): mild performance increase for 64-bit non-x86 platforms.
  * (bitcoin-core/secp256k1#1002): Make aux_rnd32==NULL behave identical to 0x0000..00 (which impacts BIP341/BIP342 signing in Bitcoin Core, making it more strictly BIP340 compliant, though not in a manner that affects security).

ACKs for top commit:
  fanquake:
    ACK 314195c8be3bd7db0d5817c4fb3aa85c84363ce9 - this includes a nice simplification to the lilbsecp build system (and thus our build system), and fixes issues like #22854. Did a Guix build on x86 (above), as well as a build on arm64 (except for the arm64 host):

Tree-SHA512: 0e048390fc148fbbdf5b98d9cce8c71067564e7d69d97b68347808a9bc45a04f4fc653c392c880d79d5d8b9cf282195520955581ac4f1595f6a948080cf5949d

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