Resolve compilation issue with Elixir 1.15

[kianmeng]

List of changes:

• Bump x509 dep (0.8.3 => 0.8.7), see https://github.com/voltone/x509/pull/60
• Bump Elixir (1.14.5 => 15.1.1) and GitHub Actions

$ mix test
==> file_system
Compiling 7 files (.ex)
Generated file_system app
==> mime
Compiling 2 files (.ex)
Generated mime app
==> x509
Compiling 22 files (.ex)
warning: Logger.warn/1 is deprecated. Use Logger.warning/2 instead
  lib/x509/test/suite.ex:585: X509.Test.Suite.sni_handler/2

warning: Logger.warn/1 is deprecated. Use Logger.warning/2 instead
  lib/x509/test/crl_server.ex:110: X509.Test.CRLServer.flush_headers/3

== Compilation error in file lib/x509/asn1.ex ==
** (UndefinedFunctionError) function :epp_dodger.parse_file/1 is undefined (module :epp_dodger is not available)
    :epp_dodger.parse_file(~c"/home/foobar/.asdf/installs/erlang/25.3.2.3/lib/public_key-1.13.3/include/OTP-PUB-KEY.hrl")
    lib/x509/asn1/oid_import.ex:19: X509.ASN1.OIDImport.get_oids/1
    lib/x509/asn1.ex:84: (module)
could not compile dependency :x509, "mix compile" failed. Errors may have been logged above. You can recompile this dependency with "mix deps.compile x509 --force", update it with "mix deps.update x509" or clean it with "mix deps.clean x509"

[kianmeng]

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