Rename FLAGD_KEEP_ALIVE_TIME to FLAGD_KEEP_ALIVE_TIME_MS

[toddbaert]

This env_var name is unclear, and inconsistent with the DEADLINE. We should rename it to FLAGD_KEEP_ALIVE_TIME_MS but also still support the old env var.

See:

java-sdk-contrib/providers/flagd/src/main/java/dev/openfeature/contrib/providers/flagd/Config.java

Line 36 in c598d9f

static final String KEEP_ALIVE_ENV_VAR_NAME = "FLAGD_KEEP_ALIVE_TIME";

[alexandraoberaigner]

Hi @toddbaert, I'd like to take this