[css-animations-1] Upgrade old bug links

[svgeesus]

These don't block publication, but should be fixed in the ED to link to the bug on GitHub::

Warning
Link for "Bug 14713" is pointing to unreliable domain or legacy service "https://www.w3.org/Bugs/Public/show_bug.cgi?id=14713".
Warning
Link for "Bug" is pointing to unreliable domain or legacy service "https://www.w3.org/Bugs/Public/show_bug.cgi?id=15251".
Warning
Link for "Bug" is pointing to unreliable domain or legacy service "https://www.w3.org/Bugs/Public/show_bug.cgi?id=14785".
Warning
Link for "Bug" is pointing to unreliable domain or legacy service "https://www.w3.org/Bugs/Public/show_bug.cgi?id=14774".
Warning
Link for "Bug" is pointing to unreliable domain or legacy service "https://www.w3.org/Bugs/Public/show_bug.cgi?id=14774".
Warning
Link for "Bug" is pointing to unreliable domain or legacy service "https://www.w3.org/Bugs/Public/show_bug.cgi?id=14786".

[tabatkins]

Do we have any stable way to refer to those, or do they just need to be moved to GitHub?

[svgeesus]

They need to be ported over. "Unstable" here means "we will switch this off at some point".

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