v2.0: Runtime: Core BPF Migration: Add checks for executable program account (backport of #2483)

[mergify]

Problem

When the Core BPF migration module was being added to the runtime, the executable flag for accounts was being deprecated, so checks for this flag were not added to the mechanisms for migrating and upgrading Core BPF programs. See #309.

These changes were reverted, and the Core BPF migration code must be aware of the executable flag on accounts.

Additionally, even though the program can still be invoked via instruction, the program account must be set as executable when performing a migration, to ensure all executable flag checks are successful on the new program.

Summary of Changes

Two small changes:

• Add a check for executable when loading an existing Core BPF program in preparation for an upgrade.
• Manually set the program account to executable: true when performing a migration from builtin to BPF.

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  This is an automatic backport of pull request Runtime: Core BPF Migration: Add checks for executable program account #2483 done by Mergify.

[buffalojoec]

Requesting backport to 2.0 to ensure any upcoming feature gates to migrate programs will allow those programs to have the executable flag set to the expected value post-migration. Note: These feature gates are not yet scheduled.