TLS: remove support for Secure Transport and BearSSL

These libraries do not support TLS 1.3 and have been marked for removal
for over a year. We want to help users select a TLS dependency that is
future-proof and reliable, and not supporting TLS 1.3 in 2025 does not
infer confidence. Users who build libcurl are likely to be served better
and get something more future-proof with a TLS library that supports
1.3.

Closes #16677

docs/INSTALL.md

@@ -137,13 +137,11 @@ alter it, you can select how to deal with each individual library.
 These options are provided to select the TLS backend to use.
 
  - AmiSSL: `--with-amissl`
- - BearSSL: `--with-bearssl`
  - GnuTLS: `--with-gnutls`.
  - mbedTLS: `--with-mbedtls`
  - OpenSSL: `--with-openssl` (also for BoringSSL, AWS-LC, LibreSSL, and quictls)
  - rustls: `--with-rustls`
  - Schannel: `--with-schannel`
- - Secure Transport: `--with-secure-transport`
  - wolfSSL: `--with-wolfssl`
 
 You can build curl with *multiple* TLS backends at your choice, but some TLS
@@ -396,85 +394,6 @@ support the legacy handshakes and algorithms used by those versions. If you
 are using curl in one of those earlier versions of Windows you should choose
 another SSL backend such as OpenSSL.
 
-# Apple Platforms (macOS, iOS, tvOS, watchOS, and their simulator counterparts)
-
-On modern Apple operating systems, curl can be built to use Apple's SSL/TLS
-implementation, Secure Transport, instead of OpenSSL. To build with Secure
-Transport for SSL/TLS, use the configure option `--with-secure-transport`.
-
-When Secure Transport is in use, the curl options `--cacert` and `--capath`
-and their libcurl equivalents, are ignored, because Secure Transport uses the
-certificates stored in the Keychain to evaluate whether or not to trust the
-server. This, of course, includes the root certificates that ship with the OS.
-The `--cert` and `--engine` options, and their libcurl equivalents, are
-currently unimplemented in curl with Secure Transport.
-
-In general, a curl build for an Apple `ARCH/SDK/DEPLOYMENT_TARGET` combination
-can be taken by providing appropriate values for `ARCH`, `SDK`, `DEPLOYMENT_TARGET`
-below and running the commands:
-
-```bash
-# Set these three according to your needs
-export ARCH=x86_64
-export SDK=macosx
-export DEPLOYMENT_TARGET=10.8
-
-export CFLAGS="-arch $ARCH -isysroot $(xcrun -sdk $SDK --show-sdk-path) -m$SDK-version-min=$DEPLOYMENT_TARGET"
-./configure --host=$ARCH-apple-darwin --prefix $(pwd)/artifacts --with-secure-transport
-make -j8
-make install
-```
-
-With CMake:
-
-```bash
-cmake . \
-  -DCMAKE_OSX_ARCHITECTURES=x86_64 \
-  -DCMAKE_OSX_DEPLOYMENT_TARGET=10.8 \
-  -DCMAKE_OSX_SYSROOT="$(xcrun --sdk macosx --show-sdk-path)"
-```
-
-The above command lines build curl for macOS platform with `x86_64`
-architecture and `10.8` as deployment target.
-
-Here is an example for iOS device:
-
-```bash
-export ARCH=arm64
-export SDK=iphoneos
-export DEPLOYMENT_TARGET=11.0
-
-export CFLAGS="-arch $ARCH -isysroot $(xcrun -sdk $SDK --show-sdk-path) -m$SDK-version-min=$DEPLOYMENT_TARGET"
-./configure --host=$ARCH-apple-darwin --prefix $(pwd)/artifacts --with-secure-transport
-make -j8
-make install
-```
-
-With CMake (3.16 or upper recommended):
-
-```bash
-cmake . \
-  -DCMAKE_SYSTEM_NAME=iOS \
-  -DCMAKE_OSX_ARCHITECTURES=arm64 \
-  -DCMAKE_OSX_DEPLOYMENT_TARGET=11.0
-```
-
-Another example for watchOS simulator for macs with Apple Silicon:
-
-```bash
-export ARCH=arm64
-export SDK=watchsimulator
-export DEPLOYMENT_TARGET=5.0
-
-export CFLAGS="-arch $ARCH -isysroot $(xcrun -sdk $SDK --show-sdk-path) -m$SDK-version-min=$DEPLOYMENT_TARGET"
-./configure --host=$ARCH-apple-darwin --prefix $(pwd)/artifacts --with-secure-transport
-make -j8
-make install
-```
-
-In all above, the built libraries and executables can be found in the
-`artifacts` folder.
-
 # Android
 
 When building curl for Android you can you CMake or curl's `configure` script.