remove openvpn master from CI test

[JacobBarthelmeh]

Testing against master branch of openvpn seems unstable for CI tests. Latest issues is:

ssl_openssl.c: In function ‘cert_verify_callback’:
ssl_openssl.c:321:5: warning: implicit declaration of function ‘X509_STORE_CTX_set0_crls’; did you mean ‘X509_STORE_CTX_get0_cert’? [-Wimplicit-function-declaration]
  321 |     X509_STORE_CTX_set0_crls(ctx, session->opt->ssl_ctx->crls);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~
      |     X509_STORE_CTX_get0_cert
  CCLD     openvpn
/usr/bin/ld: ssl_openssl.o: in function `cert_verify_callback':

[Copilot]

Pull request overview

Removes testing OpenVPN against the upstream master branch in CI to avoid instability from OpenVPN’s changing dependencies/APIs, while keeping coverage on the OpenVPN 2.6 release line.

Changes:

• Drop master from the OpenVPN build/test matrix.
• Continue testing against release/2.6 and v2.6.19.

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[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• gcc-arm-cortex-m4
• gcc-arm-cortex-m4-baremetal
• gcc-arm-cortex-m4-min-ecc
• gcc-arm-cortex-m4-tls12