Rename old TWITTER_CLIENT_ID(and SECRET) to new X_CLIENT_ID naming convention

[honzapatCZ]

What kind of change does this PR introduce?

Bug fix

Why was this change needed?

Inconsistency

Other information:

Took me quite a while to figure out why, the X integration wasn't working. These variables are nowhere documented, and as it seems also outdated.

Checklist:

Put a "X" in the boxes below to indicate you have followed the checklist;

• [x] I have read the CONTRIBUTING guide.
• [x] I checked that there were not similar issues or PRs already open for this.
• [x] This PR fixes just ONE issue (do not include multiple issues or types of change in the same PR) For example, don't try and fix a UI issue and include new dependencies in the same PR.

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[wildberry-source]

mods please approve this commit, much needed

[nevo-david]

We can't do it as it is making a breaking change. Please do it like this:

      clientId: process.env.TWITTER_CLIENT_ID! || process.env.X_CLIENT_ID!,
      clientSecret: process.env.TWITTER_CLIENT_SECRET! || process.env.X_CLIENT_SECRET!

[honzapatCZ]

Sure, done :)

[wildberry-source]

@nevo-david please commit this

[wildberry-source]

@nevo-david please approve this commit

[jamesread]

This looks fine, merged. Please could you also update https://github.com/gitroomhq/postiz-docs/blob/main/pages/providers/x.mdx as well please?