cmake: add BORINGSSL_VERSION configuration variable, test in CI

To simplify setting BoringSSL version, using:
`-DBORINGSSL_VERSION=0.20260211.0`
or
`-DBORINGSSL_VERSION=${boringssl_version}`

Previously it could be set via C flags, using complicated shell quotes:
`-DCMAKE_C_FLAGS="-DCURL_BORINGSSL_VERSION=\\\"${boringssl_version}\\\""`
(the C flags method remains, also for autotools)

It'd be nice if BoringSSL published its version not just via
`MODULE.bazel` in its source tree, but from its public headers, to make
these workarounds unnecessary.

Also:
- GHA/http3-linux: test both options.

Closes #20571

docs/INSTALL-CMAKE.md

@@ -406,6 +406,7 @@ Details via CMake
 - `AMISSL_INCLUDE_DIR`:                     Absolute path to AmiSSL include directory.
 - `AMISSL_STUBS_LIBRARY`:                   Absolute path to `amisslstubs` library.
 - `AMISSL_AUTO_LIBRARY`:                    Absolute path to `amisslauto` library.
+- `BORINGSSL_VERSION`:                      Set BoringSSL version for `curl -V`/`curl --version` output.
 - `BROTLI_INCLUDE_DIR`:                     Absolute path to brotli include directory.
 - `BROTLICOMMON_LIBRARY`:                   Absolute path to `brotlicommon` library.
 - `BROTLIDEC_LIBRARY`:                      Absolute path to `brotlidec` library.