docs: update SECURITY.md version table (1.2.x EOL, 2.0.x beta status)

[mr-raj12]

The SECURITY.md version table on master is outdated:

1. 1.2.x is marked as security-supported, but the 1.2.9 release notes
   state: "This is likely the final 1.2.x release, please migrate to using 1.4.x." It should be marked as EOL.

2. 2.0.x says "(not released)" but there have been 20 beta releases (up to 2.0.0b20). While no stable release exists yet, "(not released)" is misleading. "(beta)" would be more accurate.

Proposed updated table:

Version	Supported
2.0.x	❌ (beta)
1.4.x	✅
1.2.x	❌ (EOL)
1.1.x	❌
< 1.1	❌

Note: The 1.4-maint branch SECURITY.md also lists 1.2.x as supported and could benefit from a similar update in a follow-up.

[ThomasWaldmann]

Well, the 1.2.x situation is not that simple.

While I rather will not make any more 1.2.x releases, it could be that I (or somebody else) backport critical fixes to 1.2-maint branch, so dist package maintainers can pick them from there.

[mr-raj12]

Thanks for clarifying! I wasn't aware critical fixes might still be backported to 1.2-maint. Would something like ❌ (no new releases, critical fixes may still be backported) work better for 1.2.x? Or would you prefer keeping it as ✅ given that possibility? Happy to update the PR and issue to whatever wording you think fits best.

[ThomasWaldmann]

sounds good: ❌ (no new releases, critical fixes may still be backported)

(this is because 1.2 and 1.4 code is very similar, so backporting is no big effort)